Set Ceph pg_num dynamically based on fuel_settings

Depends on the following fuel-web commit:
https://review.openstack.org/82961

Change-Id: I7095d2600b279c1225c926e61183ed0e81180a73
Closes-bug: #1294579
Signed-off-by: Dmitry Borodaenko <dborodaenko@mirantis.com>