Added tests for Modify User Call
Added tests for Modify User Call

* Added line in guestagent fake to clear user grants after delete
* Added negative tests for Modify User

Change-Id: I16a6a9ebab281688c2a88c0c228e80337e05886e

fabian4/trove,denismakogon/trove-guestagent,denismakogon/trove-guestagent,fabian4/trove,citrix-openstack/build-trove,changsimon/trove,mmasaki/trove,citrix-openstack-build/trove,citrix-openstack-build/trove,openstack/trove,redhat-openstack/trove,cp16net/trove,zhangg/trove,changsimon/trove,redhat-openstack/trove,fabian4/trove,openstack/trove,mmasaki/trove,citrix-openstack/build-trove,citrix-openstack/build-trove,zhangg/trove,citrix-openstack-build/trove,zhujzhuo/openstack-trove,zhujzhuo/openstack-trove,hplustree/trove,changsimon/trove,cp16net/trove,mmasaki/trove,cp16net/trove,hplustree/trove,zhujzhuo/openstack-trove,redhat-openstack/trove