storage.misc: Move execCmd and its dependencies to vdsm.utils

The original vdsm.utils.execCmd actually imports and invokes
storage.misc.execCmd, which means if a process does not have
/usr/share/vdsm in its PYTHONPATH and want to use vdsm.utils.execCmd, it
will fail. Furthermore, lib/vdsm/utils.py should not use anything that
is not present in the python standard library or in a dependency of the
vdsm-python package.

execCmd is very useful for modules in vdsm-tool, currently these modules
use ad-hoc execCmd implementation. This patch moves storage.misc.execCmd
and its dependencies to vdsm.utils, so that other modules can import the
'official' execCmd implementation without adding /usr/share/vdsm to
PYTHONPATH.

Change-Id: Id8fb42035b24bc915aaa700ab128580bdb609d9e
Signed-off-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/14221
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>