qa/ceph_manager: make it possible to reuse few methods

qa/ceph_manager: make it possible to reuse few methods

Make minor adjustments to ceph_manager.CephManager so that methods
run_ceph_w(), run_cluster_cmd() raw_cluster_cmd() and
raw_cluster_cmd_result() can be reused, instead of duplicating, in
subclasses. The adjustments are -

* Having variables contain arguments that'll be prepended to every
  command received by the methods above.
* Grouping variables that needs to be overridden together so that it is
  easy to spot and override them for users.

Signed-off-by: Rishabh Dave <ridave@redhat.com>