Fixes#2515
This implements simple HTTP API which should cover same methods as IPMI
methods in Sidero.
Examples:
```
$ curl http://172.20.0.1:34791/status
{"PoweredOn":false}
```
```
$ curl -X POST http://172.20.0.1:34791/poweroff
```
API listens on bridge address, each VM has unique port which can be
found in cluster state as `apiport: NNNN`.
Signed-off-by: Andrey Smirnov <smirnov.andrey@gmail.com>
