qa/cephfs: allow not aborting execution when mount command fails

qa/cephfs: allow not aborting execution when mount command fails

This commit adds a new argument check_status to mount methods of
KernelMount, FuseMount, LocalKernelMount and LocalFuseMount. When value
of this argument is False, these methods would catch the
CommandFailedError exception and would return a tuple consisting of the
exception itself, and stdout and stderr of the mount command. This
allows reusing these mount methods while running negative tests for
commands.

The name "check_status" is selected so since teuthology's run() and
vstart_runner's run() use a variable with same name for the very same
purpose.

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