onramp: Consolidate the shell code

Move all the external command execution to a single function for clarity

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
onramp: Add flush operation for iptables

Make sure that on startup our nat table is in a known state

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
onramp: Return external addresses of deleted pods to free pool

When a pod goes away we should recover its external ip for later use

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