Small changes and fixes in ganeti-watcher.
Small changes and fixes in ganeti-watcher.

- Use constants for keys.
- Fix bug through which automatic instance restarts wouldn't be limited

Reviewed-by: iustinp


ganeti/ganeti,ganeti/ganeti,apyrgio/snf-ganeti,mbakke/ganeti,onponomarev/ganeti,yiannist/ganeti,yiannist/ganeti,ganeti-github-testing/ganeti-test-1,andir/ganeti,apyrgio/snf-ganeti,bitemyapp/ganeti,grnet/snf-ganeti,mbakke/ganeti,ganeti-github-testing/ganeti-test-1,bitemyapp/ganeti,andir/ganeti,dimara/ganeti,ganeti/ganeti,leshchevds/ganeti,apyrgio/ganeti,onponomarev/ganeti,leshchevds/ganeti,grnet/snf-ganeti,apyrgio/ganeti,andir/ganeti,yiannist/ganeti,leshchevds/ganeti,mbakke/ganeti,dimara/ganeti