qa/tasks/ceph: provide configuration for setting configs via mon

qa/tasks/ceph: provide configuration for setting configs via mon

These configs may be set using:

ceph:
  cluster-config:
    entity:
      foo: bar

same as the current:

ceph:
  config:
    entity:
      foo: bar

The configs will be set in parallel using the `ceph config set` command.

The main benefit here is to avoid using the ceph.conf to set configs which
cannot be overriden using subsequent `ceph config` command. The only way to
override is to change the ceph.conf in the test (yuck) or the admin socket
(which gets reset when the daemon restarts).

Finally, we can now exploit the `ceph config reset` command will let us
trivially rollback config changes after a test completes. That is exposed
as the `ctx.config_epoch` variable.

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>