Modify cloudinit script for MAAS provider

This PR modifies cloudinit upstart scripts, so that network configuration files in MAAS environment has the same structure as in the other environments.

Current configuration:
/etc/network/interfaces:
...
auto eth0
source /etc/network/eth0.config

/etc/network/eth0.config:
iface eth0 inet manual
auto br0
iface br0 inet dhcp
  bridge_ports eth0

New configuration:
/etc/network/interfaces:
...
source /etc/network/interfaces.d/*.cfg

/etc/network/interfaces.d/eth0.cfg:
auto eth0
iface eth0 inet manual

/etc/network/interfaces.d/br0.cfg:
auto br0
iface br0 inet dhcp