Fix import log_handler error with publish_errors set

Sync latest module log_handler from Oslo.
Commit Id is 2843a92a0e4007f50d5ad63d51fe574e1ba5e280

This handler sends the error to configured notifier.
The notifier could be any kind (log, RPC, no_op, None).

Note: when using publish_errors you also need to set the
notification_driver option too.

Closes-bug: #1240349
Change-Id: I4f759b049b8f34904ba11382a2e8fc1578677a8f
Convert RBD unit tests to use mock instead of mox

Change-Id: If32e14a8f5ff180b6c0ff46797bcb81b21f805f1
Closes-bug: 1261864
Updated error messages for volume clone

Updated error messages for volume clone from image,
snapshot and volume to be consistent across.
Updated error messages to indicate the unit (GB).

Change-Id: I9f51ad0c9ce40864a838fca06583d7d4283e61d8
Closes-Bug: #1244609
Merge "Updated error messages for volume clone"
Merge "Fix import log_handler error with publish_errors set"
Merge "Convert RBD unit tests to use mock instead of mox"