qa/workunits/rbd: update to mirror group snapshot tests

qa/workunits/rbd: update to mirror group snapshot tests

Added new environment variable RBD_MIRROR_GLOBAL_DEBUG to allow additional debug to be turned on
after a cluster has been created.

Updated tests:
• test_group_rename - disabled some checks that are failing as the fix is deferred
• test_empty_group and test_empty_groups - added some snapshot, demote, promote steps

New tests:
• test_remote_namespace - Added new scenarios that test default and non-default
  namespaces on the local and remote clusters.

Enabled tests:
- test_force_promote
- test_group_rename

Signed-off-by: John Agombar <agombar@uk.ibm.com>