Fixed timeout logic when attaching/detaching volumes.

Fixed timeout logic when attaching/detaching volumes.

The timeout logic in nova_utils.attach_volume() and detach_volume()
was not correct which may have been the root cause behind the issue
FUNCTEST-927. Timeout in both attach and detach is no longer
optional.

Also added a test to attach and detach without timeout as that path
was never tested. Updated associated test documentation as well.

JIRA: SNAPS-263
JIRA: FUNCTEST-927

Change-Id: Iea3aeab59c378917fbd175d673113e8d30e2e4b9
Signed-off-by: spisarski <s.pisarski@cablelabs.com>