core: Fixes StackOverflowError during fencing

Fixes StackOverflowError appeared during execution of fencing operation.
This fix removes commandContex instance and overriden getContext() from
RestartVdsCommand, because command context in CommandBase should be set
always during creation of command and successful locks aquiring is
checked prior to executeCommand call in CommandBase.

Also when RestartVdsCommand and its inherited commands are using global
command context, VdsKdumpDetection command executed from
VdsNotRespondingTreatmentCommand should also use global command context
and not its copy.

The overriden getContext() has been introduced in
Iaf021010188e3c3662fbf8a057da4c58f2600c02.

Change-Id: Icbe5ad9194b19a390f45c99198087a9e5b68eb9c
Bug-Url: https://bugzilla.redhat.com/1124141
Signed-off-by: Martin Perina <mperina@redhat.com>