onramp: Allow runtime specification of external interface and ip address list

We need to allow specification of external ip addresses to allocate for use as
well as the external interface.  This allows us to do that on the command line

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
onramp: Automatically enable forwarding, and support deletion of mappings

The internal and external interfaces need to have forwarding enabled, so onramp
ensures that now.

This patch also supports the removal of mapping in response to the deletion of a
pod.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
onramp: Fix podMonitor exit condition

Exit the podmonitor when the length of the podNameList becomes 0

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
onramp: Refine iptables setup script

Tweak the iptables script so that MASQUERADING rules apply only to the pod to
which traffic being inspected is destined

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>