Enrich constraints for MAAS cloud

There are a few constraints we miss when deploying machines on a MAAS cloud:
- pool: we would like to be able to set the pool constraint, similar to the zone. I could not find out how to do that.
- specify and OR operator between tags: I know it is possible to specify multiple tags, that should all be present on the machine, and also negate tags (that should not be present). But sometimes it would be handy to have an OR operator as well: constraints: tags=foo,bar|baz,^bla. In this case it should match all machines that have "foo AND (bar OR baz) AND NOT bla" tags.