Add a lower limit to oslosphinx

Starting in the 2.2.0.0a2 version of oslosphinx, a new theme
option is introduced to incubated projects.
For instance, in this patch: https://review.openstack.org/#/c/111899/
for Barbican, we should really be using the incubated option.

Change-Id: I0504c852754b08c002800a7c9685c4400e1efd77
Merge "Add a lower limit to oslosphinx"