Add External Network PNF Tests

Add External Network PNF Tests

Two changes in this commit:
 1. Add external network PNF test - this uses ttl=1
    to make sure PNF connectivity is not traversing any routers
    (in our case, the external gateway)

 2. Change current external connectivity tests to use "internet address"
    this change is required because since the PNF commit support, pings
    originating form external gateway IP are not traversing the default
    gw rules, so this change is required in order to maintain the
    original coverage of these tests.

Depends On: https://git.opendaylight.org/gerrit/54405

Change-Id: I021c3a13b73283d844d614e06a3edb2b9d867f88
Signed-off-by: Tomer Pearl <tomer.pearl@hpe.com>