slapproxy: Add minimal hateoas support for cli

Add the minimum hateoas support required to let slapproxy answer 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 a few small bugs and python3 incompatibilities in slap/hateoas.