Allow deployer to customize openstack pools

Allow deployer to customize openstack pools

By overriding the openstack_pools variable introduced by this commit, the
deployer may choose not to create some of the openstack pools, or to add
new pools which were not foreseen by ceph-ansible, e.g. for a gnocchi
storage backend.

For backwards compatibility, we keep the openstack_glance_pool,
openstack_cinder_pool, openstack_nova_pool and
openstack_cinder_backup_pool variables, although the user may now choose
to specify the pools directly as dictionary literals inside the
openstack_pools list.