Explicitly set 'builders' option
Explicitly set 'builders' option

An upcoming release of pbr will require explicitly stating which
builders are requested, rather than defaulting to html and man. Head off
any potential impact this may cause by explicitly setting this
configuration now.

Change-Id: I3ebd7d2086a47bcedc6fb35ca90bc1bb6e077250

openstack/python-heatclient