test_cephfs_shell: add a method to get command output

test_cephfs_shell: add a method to get command output

Let run_cephfs_shell_cmd() return the object instead of output in string
format so that stderr, return value, etc. of the command executed
remain available. And add a new method that returns only output of the
CephFS shell command executed.

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