Expose swift_proxy_net_ip parameter

We currently don't expose the swift_proxy_net_ip parameter
in our sample swift proxy node defintion.  It defaults to
$::ipaddress_eth0 which is very likely to break in a lot of common
deployment scenarios that we're seeing.  This commit adds it and
defaults it to $swift_proxy_address.

Fixes: bug #1213230
Merge pull request #150 from CiscoSystems/bugs/1213230

Expose swift_proxy_net_ip parameter