docs: Update TESTING.rst with gate information about ovs
docs: Update TESTING.rst with gate information about ovs

We used to compile from source for functional and fullstack jobs. For
functional job we skip single test and avoid compiling. We still do
compile from source for fullstack job. This patch adds justification to
TESTING.rst document.

Change-Id: I60c272bba6fa9eeefa29dfa4faa69433989efde2

noironetworks/neutron,noironetworks/neutron,openstack/neutron,eayunstack/neutron,huntxu/neutron,eayunstack/neutron,huntxu/neutron,openstack/neutron,mahak/neutron,mahak/neutron,openstack/neutron,mahak/neutron