Single tenant mode creates all DBaaS resources in a given tenant,
rather than the default, which is to use the users own tenant.
It relies on the nova_proxy_admin_* vars for this.
This commit adds a new trove::single_tenant_mode variable, when set
to true, sets up the remote_(nova|cinder|neutron)_client config
vars in the each of the config files for this mode to be enabled.
Change-Id: Ic59bd227fff893c705931ced3add636844533d35