Support single argument version of sqlContext.getConf
Support single argument version of sqlContext.getConf

## What changes were proposed in this pull request?

In Python, sqlContext.getConf didn't allow getting the system default (getConf with one parameter).

Now the following are supported:
```
sqlContext.getConf(confName)  # System default if not locally set, this is new
sqlContext.getConf(confName, myDefault)  # myDefault if not locally set, old behavior
```

I also added doctests to this function. The original behavior does not change.

## How was this patch tested?

Manually, but doctests were added.

Author: mathieu longtin <mathieu.longtin@nuance.com>

Closes #12488 from mathieulongtin/pyfixgetconf3.

debugger87/spark,aray/spark,wangmiao1981/spark,JerryLead/spark,mdespriee/spark,ptkool/spark,Aegeaner/spark,JoshRosen/spark,kissmetrics/spark,tengpeng/spark,rezasafi/spark,maropu/spark,lvdongr/spark,andrewor14/spark,yanboliang/spark,debugger87/spark,zhouyejoe/spark,nchammas/spark,sahilTakiar/spark,jiangxb1987/spark,ueshin/apache-spark,stanzhai/spark,szhem/spark,huang1900/spark,MLnick/spark,lxsmnv/spark,aokolnychyi/spark,facaiy/spark,mike0sv/spark,markhamstra/spark,liyichao/spark,BryanCutler/spark,WindCanDie/spark,someorz/spark,ericvandenbergfb/spark,nlalevee/spark,sachintyagi22/spark,kissmetrics/spark,xuanyuanking/spark,1haodian/spark,ahnqirage/spark,huang1900/spark,zuotingbing/spark,bravo-zhang/spark,pgandhi999/spark,witgo/spark,bdrillard/spark,techaddict/spark,bOOm-X/spark,sachintyagi22/spark,shaneknapp/spark,holdenk/spark,goldmedal/spark,huang1900/spark,wzhfy/spark,narahari92/spark,JerryLead/spark,dbtsai/spark,narahari92/spark,cloud-fan/spark,UndeadBaneGitHub/spark,guoxiaolongzte/spark,metamx/spark,zero323/spark,dhruve/spark,gioenn/xSpark,dhruve/spark,kiszk/spark,publicRoman/spark,jiangxb1987/spark,mzl9039/spark,poffuomo/spark,xflin/spark,ajaysaini725/spark,tengpeng/spark,SHASHANKB/spark,ConeyLiu/spark,apache-spark-on-k8s/spark,kimoonkim/spark,saltstar/spark,wangyum/spark,saltstar/spark,rednaxelafx/apache-spark,SHASHANKB/spark,zzcclp/spark,Panos-Bletsos/spark-cost-model-optimizer,hvanhovell/spark,MLnick/spark,liutang123/spark,zzcclp/spark,saturday-shi/spark,WeichenXu123/spark,byakuinss/spark,eyalfa/spark,minixalpha/spark,techaddict/spark,zero323/spark,hhbyyh/spark,gioenn/xSpark,icexelloss/spark,jrshust/spark,byakuinss/spark,xuanyuanking/spark,ueshin/apache-spark,panfengfeng/spark,matthewfranglen/spark,ibm-research-ireland/sparkoscope,BryanCutler/spark,Panos-Bletsos/spark-cost-model-optimizer,rikima/spark,ron8hu/spark,WeichenXu123/spark,apache-spark-on-k8s/spark,wangmiao1981/spark,MLnick/spark,bravo-zhang/spark,zero323/spark,shubhamchopra/spark,guoxiaolongzte/spark,hhbyyh/spark,apache/spark,sureshthalamati/spark,poffuomo/spark,minixalpha/spark,stanzhai/spark,hvanhovell/spark,rezasafi/spark,ueshin/apache-spark,vinodkc/spark,tengpeng/spark,eyalfa/spark,holdenk/spark,metamx/spark,spark-test/spark,spark-test/spark,tejasapatil/spark,akopich/spark,bdrillard/spark,ajaysaini725/spark,JerryLead/spark,ptkool/spark,vinodkc/spark,nlalevee/spark,lxsmnv/spark,loneknightpy/spark,kevinyu98/spark,lvdongr/spark,narahari92/spark,brad-kaiser/spark,taroplus/spark,kimoonkim/spark,sureshthalamati/spark,hvanhovell/spark,gengliangwang/spark,liyichao/spark,bravo-zhang/spark,zuotingbing/spark,debugger87/spark,mzl9039/spark,mike0sv/spark,aosagie/spark,andrewor14/spark,akopich/spark,jlopezmalla/spark,ddna1021/spark,jianran/spark,byakuinss/spark,HyukjinKwon/spark,loneknightpy/spark,skonto/spark,1haodian/spark,apache/spark,byakuinss/spark,techaddict/spark,sureshthalamati/spark,byakuinss/spark,ioana-delaney/spark,big-pegasus/spark,mdespriee/spark,brad-kaiser/spark,hvanhovell/spark,UndeadBaneGitHub/spark,spark-test/spark,ptkool/spark,apache-spark-on-k8s/spark,WindCanDie/spark,witgo/spark,icexelloss/spark,dhruve/spark,ConeyLiu/spark,map222/spark,ioana-delaney/spark,big-pegasus/spark,BryanCutler/spark,ddna1021/spark,someorz/spark,szhem/spark,highfei2011/spark,stanzhai/spark,chuckchen/spark,icexelloss/spark,WeichenXu123/spark,goldmedal/spark,Panos-Bletsos/spark-cost-model-optimizer,narahari92/spark,cloud-fan/spark,SnappyDataInc/spark,kevinyu98/spark,nchammas/spark,tejasapatil/spark,mdespriee/spark,rednaxelafx/apache-spark,dongjoon-hyun/spark,hhbyyh/spark,mike0sv/spark,mdespriee/spark,nchammas/spark,dongjoon-hyun/spark,Aegeaner/spark,michalsenkyr/spark,hvanhovell/spark,spark-test/spark,actuaryzhang/spark,SnappyDataInc/spark,pgandhi999/spark,dongjoon-hyun/spark,xuanyuanking/spark,janewangfb/spark,JerryLead/spark,jiangxb1987/spark,techaddict/spark,rezasafi/spark,liyichao/spark,SHASHANKB/spark,ahnqirage/spark,janewangfb/spark,aosagie/spark,holdenk/spark,HyukjinKwon/spark,kissmetrics/spark,WindCanDie/spark,facaiy/spark,1haodian/spark,hhbyyh/spark,wzhfy/spark,sahilTakiar/spark,hhbyyh/spark,skonto/spark,mahak/spark,jianran/spark,BryanCutler/spark,zero323/spark,ericvandenbergfb/spark,janewangfb/spark,ConeyLiu/spark,ericvandenbergfb/spark,saltstar/spark,esi-mineset/spark,gengliangwang/spark,SHASHANKB/spark,sureshthalamati/spark,UndeadBaneGitHub/spark,kevinyu98/spark,zhouyejoe/spark,jlopezmalla/spark,eyalfa/spark,patrick-nicholson/spark,esi-mineset/spark,shaneknapp/spark,zhouyejoe/spark,metamx/spark,jkbradley/spark,zzcclp/spark,bOOm-X/spark,shubhamchopra/spark,metamx/spark,kevinyu98/spark,facaiy/spark,mahak/spark,JoshRosen/spark,brad-kaiser/spark,rezasafi/spark,actuaryzhang/spark,liutang123/spark,mzl9039/spark,szhem/spark,Aegeaner/spark,ioana-delaney/spark,srowen/spark,actuaryzhang/spark,ddna1021/spark,ahnqirage/spark,ajaysaini725/spark,guoxiaolongzte/spark,JerryLead/spark,kimoonkim/spark,kiszk/spark,cloud-fan/spark,brad-kaiser/spark,caneGuy/spark,nla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