Don't test Xenial agent (#1483)
Don't test Xenial agent (#1483)

Agents don't support python 3.5 any more because they use fstrings.
Therefore, let's not test this- and it's in extended support so
we might want to reconsider which Ubuntu versions we test.
cloudify-cosmo/cloudify-system-tests,cloudify-cosmo/cloudify-system-tests