Add run_as_root option to utils.execute

Add run_as_root option to utils.execute

This change adds the run_as_root option that exists in the
processutils.execute method. This option makes it possible
to just look up the root_helper from the config when
necessary, while still making it possible to not use any
privilege escalation.

Currently, processutils.execute exposes some eventlet issues
in the neutron codebase, otherwise this patch would just move
us over to using it.

Change-Id: Ic9ebe77dada4f7f044f009ff04494906d4f50ae2
Partially-Implements: blueprint rootwrap-daemon-mode