Completely refactor the team/JSON handling in libnm's NMSettingTeam and
NMSettingTeamPort.

- team handling was added as rh#1398925. The goal is to have a more
  convenient way to set properties than constructing JSON. This requires
  libnm to implement the hard task of parsing JSON (and exposing well-understood
  properties) and generating JSON (based on these "artificial" properties).
  But not only libnm. In particular nmcli and the D-Bus API must make this
  "simpler" API accessible.

- since NMSettingTeam and NMSettingTeamPort are conceptually the same,
  add "libnm-core/nm-team-utils.h" and NMTeamSetting that tries to
  handle the similar code side-by-sdie.
  The setting classes now just delegate for everything to NMTeamSetting.

- Previously, there was a very fuzzy understanding of the provided
  JSON config. Tighten that up, when setting a JSON config it
  regenerates/parses all other properties and tries to make the
  best of it. When modifying any abstr...