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: Idd28d83cb7a015fd610698a9575bc4dd0c3697d2

openstack/oslo.context