Fixes the method update_datastore
Fixes the method update_datastore

Once a datastore has been updated with a default-datastore-version
update_datastore cannot reset it back even if we call method again
as follows:
    trove-manage datastore_update 'test_ds' ''

The update has been made to reset default-datastore-version when needed.

Change-Id: Ic42bf81ab3e81376da88635a6323ef24c2f70afd
Closes-Bug: #1463292

mmasaki/trove,hplustree/trove,openstack/trove,fabian4/trove,hplustree/trove,cp16net/trove,redhat-openstack/trove,redhat-openstack/trove,openstack/trove,mmasaki/trove,zhangg/trove,cp16net/trove,mmasaki/trove,redhat-openstack/trove,fabian4/trove,zhangg/trove,cp16net/trove,fabian4/trove