Init Glance database only on the node with the database backend
Init Glance database only on the node with the database backend

Since [1] Glance init depends on either g-api or g-reg being
enabled.
This broke multinode g-api deployments with singlenode database
backend.
This commit aligns Glance with other services w.r.t when to
apply database init.

[1] d8dec362baa2bf7f6ffe1c47352fdbe032eaf20a

Change-Id: Idc07764d6ba3a828f19691f56c73cbe9179c2673
Closes-bug: #1860021

openstack-dev/devstack,nawawi/openstack,nawawi/openstack,openstack-dev/devstack