Set default value for MANILA_CEPH_GANESHA_RADOS_STORE
Set default value for MANILA_CEPH_GANESHA_RADOS_STORE

It's not a required configuration item; and its not required
with cephadm deployed NFS-Ganesha daemon/s.

Change-Id: I54380f1cb905dfa5ab287ba423561aa75bc1d2f4
Signed-off-by: Goutham Pacha Ravi <23768c94a9ff9c3948f2b1373d75b7bec9121063@gmail.com>

openstack/devstack-plugin-ceph