Add specific doc build option to tox

Add doc venv to align with other OpenStack projects (such as keystone,
heat and nova-specs) who are using 'docs' for there doc builds.

Change-Id: I37771db339d864b2b938d1c02d15d38b07093493
On rebuild check for null image_ref

While parsing the body passed in a rebuild request there is no check to
ensure that image_ref is not None.  This leads to a raised exception and
500 response by the API.

Change-Id: Ic9e51a761d0c1f30d55f4a167c876a6aa4f00a41
Closes-bug: 1312278
Merge "On rebuild check for null image_ref"
Merge "Add specific doc build option to tox"