Consistency checks for config wrt disk templates

This patch adds consistency checks for the configuration regarding
disk templates. In particular, it checks that the list of enabled
disk templates is not empty, does not contain any bogus templates,
and that all instances use actually enabled disk templates.
Additionally, it fixes a small formatting issue regarding the list
of enabled hypervisors.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Michele Tartara <mtarara@google.com>