Take control over Rally logics

Take control over Rally logics

Rally selects the first external network [1] which could raise side
effects if:
 - another testcase is creating a dummy external network in //
 - all external subnets are not reachable from jumphost
It's now overriden by EXTERNAL_NETWORK as defined in Functest

Rally creates a new shared network if one already exists [2].
As juju now allows other shared networks, it can be safely created by
Functest to allow only one logic and to handle provider networks,
different name servers, etc...

[1] https://github.com/openstack/rally-openstack/blob/master/rally_openstack/verification/tempest/config.py#L146
[2] https://github.com/openstack/rally-openstack/blob/master/rally_openstack/verification/tempest/context.py#L85

Change-Id: Icf8c08077d4b0a9eb1c2e1b7309c62957b0a3b63
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
(cherry picked from commit ee52195f4476d3debf9c3f92c6d808256a199500)