Ignore GUI service on export.
Merge pull request #190 from frankban/env-export-gui

QA:
- make devel/debug/prod;
- deploy charms/bunldes;
- change settings, constraints, num units;
- export the environment and check the 
  resulting yaml is ok;
- refresh;
- deploy mysql;
- deploy wordpress, naming the service as "juju-gui";
- relate the two;
- export the environment and check 
  juju-gui is not included in the yaml.