Fix test_connectivity_dvr_and_no_dvr_routers_in_same_subnet test
This patch fixes couple of issues in scenario test from
1. Replace safe_client with client object
In class NetworkConnectivityTest there was used safe_client
but there is no such attribute in this class. Object "client"
should be used instead.
2. It also fixes in the same test how external network's subnet
ID is get from the network's info.
3. Change to use admin_client to get details of external
network's subnet as this subnet don't belongs to tenant user
so regular client gets 404 error while doing subnet_show
4. Check the subnets IP version to retrieve only an IPv4 one.