OS::Nova::Server depend on subnets related to nets

Nova supports creating network interfaces by network ID however
if the network and the subnet are part of the same stack as this
server then a race occurs between the creation of the subnet and
the creation of the server. Likewise for delete the subnet cannot
be deleted until all ports are removed from it.

This change adds the same add_dependencies workaround to
OS::Nova::Server that OS::Neutron::Port and OS::Neutron::RouterGateway
have.

The confirmation that this change fixes the race will be confirmed
by tempest test NeutronResourcesTestJSON once
https://review.openstack.org/#/c/82393/ lands.

Change-Id: I626b976446cda709d96823c9de331eafc29d3c68
Closes-Bug: #1243992
Merge "OS::Nova::Server depend on subnets related to nets"