When an external Ceph cluster is used, the "filesystem_store_datadir"
option is set in the "glance-api.conf" configuration file.
It should not, it's because of this condition:
{% elif enable_ceph | bool and glance_backend_ceph | bool %}
When external Ceph cluster is used "enable_ceph" is false and
"glance_backend_ceph" is true.
glance_backend_ceph variable should be enought for this condition.
Change-Id: I2a2ab420727888cfd9fcbc4bd30a08410163b36e
Closes-Bug: #1718728
c20cf01872