Do not make ::glance::backend::file inherits from ::glance::api because
::glance::api has parameters.
The class was broken until now.
Also, we need to include glance::backend::file to have
'filesystem_store_datadir' parameter in Glance config, since we're using
file backend.
If you don't provide this parameter, Glance fails to start:
glance_store._drivers.filesystem [-] Specify at least
'filesystem_store_datadir' or 'filesystem_store_datadirs' option
Change-Id: Iba6791199f7a2c7fef24b1e8752ed0b564b8512f
57009efc4a