RFR: A generic fixtures loader for loading fixtures from files. 

## What?

We basically want to move all models being defined repetitively in tests to a more sane approach where these models are available as fixtures and each test selectively saves them to db and deletes them when tests are done. 

## Why?

Less code duplication. Also, we'll have plenty of JSON/YAML samples for rules, actions, etc.

## How?

All generic fixtures that can be shared across tests should be placed in
```
st2tests/fixtures/generic
  actions/
  rules/
  executions/
 ...
```
If you really want specific fixtures for a test, you can create a fixture_pack with same structure but under a different root. Example: 
```
st2tests/fixtures/myspecialtest
  actions/
  rules/
  executions/
 ...
```
## Note

There are some fixtures that don't have data models. Examples are actionchains and workflows. Those are loaded using load_fixtures. 

I am still thinking through these. There are data model models that I might have skipped but adding them is easy and localized to one file: fixturesloader.py. 

Have a look at  aceb37e to see how this is beneficial. The test is more sane now.