Adds a new config-flags option to the charm that
supports setting a dictionary of ceph configuration
settings that will be applied to ceph.conf.

This implementation supports config sections so that
settings can be applied to any section supported by
the ceph.conf template in the charm.

Change-Id: I306fd138820746c565f8c7cd83d3ffcc388b9735
Closes-Bug: 1522375