Divide dhcp and l3 agent scheduling into separate extensions

Rationale behind this is that some plugins may support only dhcp or l3 agent scheduling.
The patch is nothing more than refactoring. Functionality was not changed.

Fixes bug 1196806

Change-Id: Ie174059adfaed3028bbf65de7bde3497dd9bc664
Merge "Divide dhcp and l3 agent scheduling into separate extensions"