Improve help message of loadbalancer commands

- Correct the help for healthmonitor type (Fixes bug #1203582)
- Use choices for the enum fields:
  healthmonitor type, pool lb-method, pool protocol
- Convert metavar to uppercase

Change-Id: I2a648949b48385249dde0c5868f564e1f17c3675
Merge "Improve help message of loadbalancer commands"