VMware: update get_available_datastores to only use clusters
VMware: update get_available_datastores to only use clusters

The ESX support was deprecated in Juno. We no longer need this
method to get the ESX's datastores. It should only read the
clusters datastores.

TrivialFix

Change-Id: I51ea9ce5327dff325b4642c6101904a66447e816
rahulunair/nova,yatinkumbhare/openstack-nova,eonpatapon/nova,zhimin711/nova,varunarya10/nova_test_latest,blueboxgroup/nova,ruslanloman/nova,cyx1231st/nova,zzicewind/nova,openstack/nova,apporc/nova,Juniper/nova,cernops/nova,barnsnake351/nova,sebrandon1/nova,gooddata/openstack-nova,fnordahl/nova,Stavitsky/nova,barnsnake351/nova,Stavitsky/nova,bigswitch/nova,Yusuke1987/openstack_template,BeyondTheClouds/nova,Francis-Liu/animated-broccoli,alaski/nova,Tehsmash/nova,CEG-FYP-OpenStack/scheduler,cernops/nova,jianghuaw/nova,belmiromoreira/nova,NeCTAR-RC/nova,j-carpentier/nova,sebrandon1/nova,mmnelemane/nova,mikalstill/nova,akash1808/nova_test_latest,scripnichenko/nova,felixma/nova,double12gzh/nova,double12gzh/nova,rahulunair/nova,fnordahl/nova,zaina/nova,shail2810/nova,mikalstill/nova,kimjaejoong/nova,jianghuaw/nova,zhimin711/nova,cloudbase/nova,rajalokan/nova,alaski/nova,noironetworks/nova,CCI-MOC/nova,JianyuWang/nova,hanlind/nova,CCI-MOC/nova,takeshineshiro/nova,openstack/nova,joker946/nova,cernops/nova,watonyweng/nova,rajalokan/nova,vmturbo/nova,rajalokan/nova,LoHChina/nova,whitepages/nova,rajalokan/nova,blueboxgroup/nova,devendermishrajio/nova,raildo/nova,LoHChina/nova,vmturbo/nova,MountainWei/nova,kimjaejoong/nova,nikesh-mahalka/nova,JioCloud/nova,shail2810/nova,openstack/nova,mandeepdhami/nova,BeyondTheClouds/nova,adelina-t/nova,akash1808/nova,yatinkumbhare/openstack-nova,cyx1231st/nova,klmitch/nova,thomasem/nova,rahulunair/nova,joker946/nova,TwinkleChawla/nova,tealover/nova,mmnelemane/nova,alvarolopez/nova,CloudServer/nova,tangfeixiong/nova,alexandrucoman/vbox-nova-driver,BeyondTheClouds/nova,petrutlucian94/nova,vmturbo/nova,dawnpower/nova,alexandrucoman/vbox-nova-driver,phenoxim/nova,dims/nova,cloudbase/nova-virtualbox,Tehsmash/nova,mandeepdhami/nova,jeffrey4l/nova,eonpatapon/nova,dims/nova,noironetworks/nova,yosshy/nova,hanlind/nova,cloudbase/nova-virtualbox,zaina/nova,edulramirez/nova,ruslanloman/nova,hanlind/nova,alvarolopez/nova,sebrandon1/nova,isyippee/nova,watonyweng/nova,TwinkleChawla/nova,apporc/nova,ted-gould/nova,whitepages/nova,varunarya10/nova_test_latest,zzicewind/nova,Yusuke1987/openstack_template,tealover/nova,Juniper/nova,jianghuaw/nova,devendermishrajio/nova_test_latest,dawnpower/nova,raildo/nova,Juniper/nova,scripnichenko/nova,petrutlucian94/nova,gooddata/openstack-nova,JioCloud/nova,bigswitch/nova,NeCTAR-RC/nova,devendermishrajio/nova,yosshy/nova,gooddata/openstack-nova,Francis-Liu/animated-broccoli,tudorvio/nova,klmitch/nova,belmiromoreira/nova,tangfeixiong/nova,vmturbo/nova,CEG-FYP-OpenStack/scheduler,takeshineshiro/nova,akash1808/nova,bgxavier/nova,isyippee/nova,nikesh-mahalka/nova,adelina-t/nova,mikalstill/nova,tudorvio/nova,cloudbase/nova,devendermishrajio/nova_test_latest,iuliat/nova,JioCloud/nova_test_latest,mahak/nova,j-carpentier/nova,bgxavier/nova,jianghuaw/nova,JianyuWang/nova,phenoxim/nova,klmitch/nova,thomasem/nova,Juniper/nova,gooddata/openstack-nova,JioCloud/nova_test_latest,cloudbase/nova,jeffrey4l/nova,mahak/nova,mahak/nova,klmitch/nova,felixma/nova,MountainWei/nova,CloudServer/nova,edulramirez/nova,akash1808/nova_test_latest,ted-gould/nova,iuliat/nova