Improve doc strings after changing method for index creation
Improve doc strings after changing method for index creation

After replacing ensure_index for create_index in impl_mongodb and impl_db2
doc strings and comments were not changed. This patch improves it.

Change-Id: I21022a054dd9c85a15111efe1723e5437ac60022

fabian4/ceilometer,eayunstack/ceilometer,cernops/ceilometer,openstack/ceilometer,idegtiarov/ceilometer,ityaptin/ceilometer,cernops/ceilometer,mathslinux/ceilometer,r-mibu/ceilometer,isyippee/ceilometer,isyippee/ceilometer,idegtiarov/ceilometer,eayunstack/ceilometer,redhat-openstack/ceilometer,redhat-openstack/ceilometer,r-mibu/ceilometer,ityaptin/ceilometer,fabian4/ceilometer,openstack/ceilometer,mathslinux/ceilometer,maestro-hybrid-cloud/ceilometer,maestro-hybrid-cloud/ceilometer