Minor addition of packages needed to contribution docs
Minor addition of packages needed to contribution docs

* Need to install libxml2-dev and libxslt-dev for tox to work properly.

Change-Id: I35a614164a5f8b2b02115624f9d54ca42b7c8d0f

gilbertpilz/solum,gilbertpilz/solum,gilbertpilz/solum,openstack/solum,julienvey/solum,ed-/solum,julienvey/solum,stackforge/solum,stackforge/solum,ed-/solum,ed-/solum,devdattakulkarni/test-solum,gilbertpilz/solum,devdattakulkarni/test-solum,ed-/solum,openstack/solum