Remove pbr warnerrors in favor of sphinx check
Remove pbr warnerrors in favor of sphinx check

This change removes the now unused "warnerrors" setting,
which is replaced by "warning-is-error" in sphinx
releases >= 1.5 [1].

[1] http://lists.openstack.org/pipermail/openstack-dev/
2017-March/113085.html

Change-Id: I573c0e7105678e09581c6d40b86c8a3102c8d882

openstack/oslotest,openstack/oslotest