Add -gdb flag to start-impalad.sh to start Impala under gdb
Add -gdb flag to start-impalad.sh to start Impala under gdb

Change-Id: I19f027680cfbf6a7cbc4b311e07f244d67ff683d
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1125
Reviewed-by: Henry Robinson <3eca10f30041813f045165784e24b5a950a6cc7e@cloudera.com>
Tested-by: jenkins

michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala