Define Interfaces and provide implementations
for draining.

Two Interfaces defined:

1. DrainRequest interface represents a single drain
   request, it provides start/stop/status of the request
   eventually driving drain kubectl API

2. Manager interface manages DrainRequests

Signed-off-by: adrianc <[email protected]>

Signed-off-by: adrianc <[email protected]>

implement handling of drain state.

- add handler function for drain state
- use drain manager to handle drain requests
- handle NodeMaintenance updates/deletion during drain
- extend controller test to cover draining state

Signed-off-by: adrianc <[email protected]>