slapproxy: Add minimal hateoas support for cli

Add the minimum hateoas support required for the cli commands:
- `slapos service list`
- `slapos service info <reference>`
- `slapos computer list`
- `slapos computer info <reference>`

To enable hateoas, complete the slapos.cfg configuration with:
```
[slapos]
master_rest_url = <proxyaddr>/hateoas
```

Also: Fix some bugs and python3 incompatibilities in slap/hateoas.py.

See merge request nexedi/slapos.core!290