Allow setting a rootwrap cmd for functional tests

Previously, sudo-requiring functional tests hardcoded the use of
'sudo' as the root helper.  Devstack gate jobs do not allow
password-less invocation of 'sudo', though, so such tests were unable
to run in the gate.  This patch adds the ability to configure the
rootwrap command installed by devstack by setting the OS_ROOTWRAP_CMD
environment variable in the test execution environment, allowing
sudo-requiring tests to run.

Change-Id: I3b8f6b4f14ac1743e08b9401f73951885165350a
Partial-bug: #1336172
Merge "Allow setting a rootwrap cmd for functional tests"