New 'config' attribute allows custom configuration

Sample usage:

class { 'ceph::conf':
  fsid => $fsid,
  config => {
    'osd max backfills' => 1,
    'osd recovery max active' => 1,
  }
}
Add rspec tests for the 'config' parameter

Testing is currently only done for ceph::conf because other rspec tests
are not yet created.
Ensure a consistent ordering of config keys

Without this patch, config key ordering was not deterministic which
leads to testing failure or potentially useless configuration file
modifications.