Add host extension

Host extension for midonet exposed as is documented here:
https://github.com/midokura/midonet-docs/blob/master/api/source/api/\
rest-api-specification.rst#host

Host Version has been merged into Host entity.

Host interface read only resource exposed too.

Full tests over exposed endpoints.

Change-Id: I990e4fec04bc853e221c609d6c404cba363284b4