HTML test report only when nosehtmloutput is installed
HTML test report only when nosehtmloutput is installed

Most horizon plugins import openstack_dashboard.test.settings
in their test settings, but they do not necessarily have nosehtmloutput
in test-reqs. Assuming nosehtmloutput potentially breaks plugins tests,
we check the availability of htmloutput module (from nosehtmloutput)
and specify --with-html-output option if available.

Change-Id: I62502d28c75f823c47edb9de83932363cc2be99f
Closes-Bug: #1697731

BiznetGIO/horizon,yeming233/horizon,openstack/horizon,ChameleonCloud/horizon,BiznetGIO/horizon,NeCTAR-RC/horizon,NeCTAR-RC/horizon,ChameleonCloud/horizon,NeCTAR-RC/horizon,openstack/horizon,noironetworks/horizon,openstack/horizon,ChameleonCloud/horizon,yeming233/horizon,openstack/horizon,noironetworks/horizon,yeming233/horizon,yeming233/horizon,noironetworks/horizon,BiznetGIO/horizon,NeCTAR-RC/horizon,noironetworks/horizon,BiznetGIO/horizon,ChameleonCloud/horizon