OVS add_ovsbridge_linuxbridge fails for missing `source` in e/n/i

contrib.networking.ovs.__init__.add_ovsbridge_linuxbridge() fails to ifup anything in /etc/network/interfaces.d/ as Juju overwrites /e/n/i with its own config and removes that line.

Expected behavior is to drop a <veth-name>.cfg into that folder and ifup the veth, however it will always fail.

Fix: Either Juju should respect the original contents of e/n/i or add_ovsbridge_linuxbridge should append 'source /etc/network/interfaces.d/*' to the end of the e/n/i before trying to add and ifup the veth.