tox: Stop building *all* docs in 'docs'
tox: Stop building *all* docs in 'docs'

The 'docs' target currently builds the documentation trees in 'api-ref'
and 'api-guide', in addition to 'doc'. This massively increases the
amount of time docs take to build both locally and in the gate. It's
not necessary for gate, since separate jobs take care of the other
documents for barbican.  As such, we should stop doing it.

For users that *do* care about this (for whatever reason) a new
'all-docs' target is included.

Change-Id: I3aa314f8643fcb4da93f8642034edddc2145bc32

openstack/monasca-log-api,stackforge/monasca-log-api,openstack/monasca-log-api,stackforge/monasca-log-api,openstack/monasca-log-api,stackforge/monasca-log-api