identity: disable trove by default

Trove (Database as a service) is still experimental and should be
disabled by default to avoid error with current default configuration.

This patchs does it by adding a new parameter trove_enabled (False by
default) and only create Keystone endpoint if this parameter is at True.

It also avoids to see the Database panel in Horizon by default.
Merge pull request #621 from enovance/disable-trove

identity: disable trove by default