Add pre-install phase for extras.d plugins

An additional call hook for the extras.d plugins that is called before
any service installation occurs turns out to be needed by Ceph and
useful for other plugins.

This is called between the installation of the system packages listed
as prerequisites and the installation of the actual services.

Change-Id: Id723bdc7542182d2b2eff40c0dc0127307f094c5
dt's changes

* Move lib/storage_plugins/ceph -> lib/ceph
* Move lib/cinder_plugins/ceph -> lib/cinder_backends/ceph
* Refactor Cinder config to use cinder-backends
* Remove unnecessary changes to other DevStack files

Change-Id: I9c75a74f1bf08ee77487f22ad611a36cffcc2551
Merge branch 'cinder-backends' into dt-test
Merge branch 'add-pre-install' into dt-test
Merge branch 'new-ceph-backend' into dt-test
Merge branch 'test-ini-improvements' into dt-test
Merge branch 'usb-prereq' into dt-test
Merge branch 'conf-tempest' into dt-test