The auto-generated local env does not work in trusty

[Francesco Banconi]

Due to bug #1306537 quickstart hangs when trying to bootstrap the initial automatically generated local env, and, more in general, any local environment not specifying "precise" as default series.

While we wait for the bug above to be fixed and the change landed, we can update quickstart in trusty to be smarter, i.e.:

- switch to "trusty" defaults:
  - the suggested default series when creating new environments is "precise" and should be changed to "trusty";
  - we ported the GUI to trusty, so now the GUI can be deployed by default from the trusty charm.

- use the bootstrap node series to select which GUI charm to deploy:
  - if the bootstrap node series is "precise", use the precise charm;
  - otherwise default to trusty;
  - only deploy to a new machine if the local provider is used or the bootstrap node series is not "precise" or "trusty".

- include the "trusty" default-series field in the initial auto-generated local env: this way the initial environment should work even before fixing core bug 1306537.