Since self.env.create always return db object now,
we can use this returned value instead of
self.env.clusters list.
It's a refactoring, so no bug or blueprint.
Change-Id: If7c84cb7124bcf08ef5ff110542012564190fae1
nailgun.errors have a huge set of exceptions but without hierarchy. This
patch remove exception generation from dict and make it explicitly with
python classes and add some exceptions hierarchy. Now all network errors
inherit from NetworkException and same for other exceptions.
Change-Id: I9a2c6b358ea02a16711da74562308664ad7aed97
Closes-bug: #1566195
Igor Gajsin