Check if dhcp port exist in adding router interface

Fix: #11
Signed-off-by: Rossella Sblendido <rossella@midokura.com>
Checking if not subnet_id in add router interface

Change-Id: I7711eff6e3a3ba80f53dbf4c02eef6543d7610ce
Signed-off-by: Rossella Sblendido <rossella@midokura.com>
Corrected Pep8 warnings

Change-Id: I0f89426c786dae83891687f9b3dd0b795dd1f896
Signed-off-by: Rossella Sblendido
Merge pull request #21 from midokura/rossella/issue11

Rossella/issue11