SheepdogDriver: Improve get_volume_stats operation

SheepdogDriver: Improve get_volume_stats operation

This patch improves get_volume_stats method to invoke
dog command in SheepdogClient Class method instead of
in SheepdogDriver class method directly. Here are two
benefits we can realize as a result:

1.The current implementation can only get volume status
by local sheepdog node, but the SheepdogClient Class also
supports the method to run dog command with remote sheepdog
node.

2.SheepdogClient Class methods are implemented to run dog
command with fine grained Error handling. So it can improve
the robustness and is more readable to run dog command in
SheepdogClient Class method.

Change-Id: I64ca193cd50e6914d0d5fb6cf711c760dfb65b8c
Closes-Bug: #1512287