Allow skipping list_services

Allow skipping list_services

Shade forces the use of admin endpoints when listing services [1].

A new env var is added to allow running Functest when only public
endpoints are reachable (e.g. testing from a VM as proposed for ONAP
integration).

It seems the best approach as list_services provides key data and
we do avoid blocking requests.

It seems easier and more generic than listing all calls in
testcases.yaml.

[1] https://github.com/openstack/shade/blob/master/shade/openstackcloud.py#L9956

Change-Id: Idcbe5d364f4230ffe1da34e7da0e99e863f15c5e
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>