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: Ia04058232a839c4eaf2c85849e8b71879ae414e7

openstack/python-designateclient