Install tempest to re-use REST client code in solum functional tests

Since code is already available in tempest project for REST client, it
is always good to re-use it. The patch [1] tries to achieve this. But it
needs to have tempest setup. So enable tempest service in devstack job.

[1] https://review.openstack.org/64059

Change-Id: I7eeaab076681107c2acab984736eca3772b69fdf
Add niceness gate for operations-guide

This is the first gate using the new openstack-doc-tools
repository.
The gate should be non-voting.

Change-Id: Ib6400d079743962889986a2694d4f2f3ce6af317
Merge "Install tempest to re-use REST client code in solum functional tests"
Merge "Add niceness gate for operations-guide"