Publish only for trunk

Publish documents only for trunk.

This patch needs to go in before we branch for stable. Once branched,
the stable/icehouse branch should be changed again to get a
publishdocs section that only contains the following line for publishdocs:
openstack-doc-test --check-build --publish --only-book install-guide --only-book config-reference

Change-Id: Ibb2f7ef3917801f158896308f1d5b5ba350acf5c
Merge "Publish only for trunk"