provider/maas: bridgescript should support vlans

[Boroczki, Lajos]

In MAAS 1.9 it is possible to set up nodes to use multiple networks separated using VLANs.
A simple MAAS deployment of a node yields the intefaces file attached as interfaces_maas.
Juju bootstrap of a different node yields the intefaces file attached as interfaces_juju, whereas the expected result is attached as interfaces_juju_expected.

Thanks for your help.