revamp sshproxy.yaml with an all-new overrides system

[cyrilst]

This is a big breaking change in sshproxy.yaml

• it now allows to override any key!
• the route system, users and groups override systems are all replaced by a new overrides system, which is more flexible and upgradeable (we can add new filters to match against).
• matches can be made against user, group and source (host:port of the listening sshd)
• multiple overrides can match, they are applied from top to bottom (meaning the last one will have precedence if a same key is overriden multiple times)

Migration example:

routes:
  service1:
    source: [host2]
    dest: [host3]
  default:
    dest: [host1]
users:
  - foo:
      debug: true

becomes:

service: default
dest: [host1]
overrides:
  - match:
      - source: host2
    service: service1
    dest: [host3]
  - match:
      - user: foo
    debug: true

A more complex config can now be done, like this:

overrides:
  - match:
      - source: host1
        group: foo
      - group: bar
        group: baz
    debug: true

Meaning "(if source is host1 AND user has group foo) OR (if user has group bar AND user has group baz) then debug is set to true"