Fixed lp:1373385 - support both positive and negative MAAS tags
Merge pull request #890 from dimitern/lp-1373385-maas-negative-tags

Fixed lp:1373385 - support both positive and negative MAAS tags

Juju now accepts both positive and negative tags constraints
and passes them to MAAS's acquireNode API properly, using the
"tags" and "not_tags" params.

Example:
$ juju deploy svc --constraints tags=virtual,^high-cpu,^high-mem

Fixes http://pad.lv/1373385