Provide the ability to enable the root user on a Trove instance

Currently there is no affordance for enabling a root user when specifying an OS::Trove::Instance resource in a Heat template (see http://docs.openstack.org/developer/heat/template_guide/openstack.html#OS::Trove::Instance-props). This blueprint is to provide such an affordance.

See https://github.com/openstack/database-api/blob/master/openstack-database-api/src/markdown/database-api-v1.md#enable-root-user-post-instancesinstance_idroot for the corresponding Trove REST API.