Close file descriptors when executing sub-processes

Pass close_fds=True to subprocess.Popen(), as is currently done in
nova.utils.execute().

Note that this folsom-stable branch fix is not a cherry-pick of the
master branch fix at https://review.openstack.org/22486 due to prior
refactoring in grizzly.

Fixes bug #1130735

Change-Id: Ia7a889b21a8069b559cb4641480abf416dfc903c