This patch adds a new config parameter NATIVE_METADATA_ROUTE to
allow users to change metadata route used for native metadata service.
The default value is 169.254.169.254/32.
This can be used as a work-around for an existing cirros bug (#1607901),
where 169.254.169.254/32 is dropped by cirros DHCP client for option 121.
By setting NATIVE_METADATA_ROUTE=169.254.169.254/31, it can be accepted
by cirros thus allowing metadata requests sent to native metadata proxy.
Change-Id: Ibb5b76487284e748a60f75cc713bc7a17c553adb
Related-Bug: #1607901