Skip searching network in project if external or shared

Skip searching network in project if external or shared

It meets the OpenStack rules:
  - any router can be connected to an external network.
  - any VM can be connected to a shared network

It completes https://gerrit.opnfv.org/gerrit/59865 which partially
skip the lookup.

Then CreateRouterSuccessTests passes when the external network is not
owned by the user running snaps_smoke.

JIRA: SNAPS-320

Change-Id: I2c60363002c6b667f90ff3f5f984eace4fa86c99
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>