onramp: Fix file permission issue with forwarding setup

when opening a file to write our forwarding config, we need to use OpenFile,
because os.Open always specifies O_RDONLY, which won't work here for obvious
reasons

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
onramp: Make internal interface configurable

Add an option to specify the internal interface for the onramp router

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