Handle dynamic forms in a cleaner and a more performance way

* Store form classes in-memory to reduce loading time between wizard
  steps
* Clear form data when hitting 'Add to Env'/'Quick Deploy' button

As a support for these changes refactor existing code a bit:
* move package attribute getters to muranodashboard.api.packages
together with their @with_cache decorators and add an important remark
about what should and what should not be cached;
* rewrite muranodashboard.catalog views and urls modules to make
deploy() and quick_deploy() the only entry points which are decorated
with @update_lastest_apps and @clear_forms_data.

Change-Id: I0fc249d168e0133cdc6f1c288a5734ac1bf7d475
Implements: blueprint separate-form-class-storage-and-form-data
Merge "Handle dynamic forms in a cleaner and a more performance way"