Add python-ceilometerclient in requirements

With the addition of the ceilometer client in the scenario tests, the
python-ceilometerclient is missing. This patch adds it to the packages
to be installed in requirements.txt.

Change-Id: Icb667def3afb71d20bc7a90e6e031cf671adc321
Closes-Bug: #1337309
Merge "Add python-ceilometerclient in requirements"
Merge branch 'master' of https://github.com/openstack/tempest