Add optional shared network for tempest testing

For some nova tests tempest creates instances without specifying
a network. As a result, neutron can't to create an interface on
external network (as in contrail env) or can't choose network if
we have several external or shared networks.
This optional network will be added to generated tempest config
only if appropriate class is included in model.

Change-Id: If91b98410f96b9d3f80fa6abbf729db13f855c44
Closes-bug: PROD-25148
2 files changed