Make swift backend not enherit glance::api.

Having the glance::backend::swift class inherit glance::api
makes it unusable (it causes puppet errors due to missing password
which is required by glance::api).

Since this module is mainly just useful to configure sets of
swift configs it can stand on its own.

Change-Id: I549087e2e9e9e1abd43bd0f78e4f4dee71490ace