922825: pre_subscribe conduit now contains more data

Data added:
- pool_id: the id of the pool from which the entitlement(s) will be consumed
- quantity: the quantity requested to consume

Created seperated slots/conduits for auto-attach
- auto-attach/healing now call pre_auto_attach and post_auto_attach
  hooks.

__Testing Notes__
Be sure to test both command line and GUI. Both manual and auto attach should be tested as well.

Install the example plugins:
```
sudo make install-example-plugins
```
Tail the log for plugin invokations:
```
tail -f /var/log/rhsm/rhsm.log | grep plugins.py
```

Register and subscribe to a single pool and auto subscribe.