Marathon-LB v1.12.0-1.8.3-beta-3

Marathon-LB based on HAProxy 1.8.3 (which is still relatively new).

Summary:
-Filter SSE events to reduce Marathon load
-Fix automated Docker build issues (use multiple gpg key servers)
-Handle some errors created by new reload mechanism.

Shortlog from beta-1 to beta-3:

git shortlog v1.12.0-1.8.3-beta-1..v1.12.0-1.8.3-beta-3
Justin Lee (3):
      Beta state fix (#538)
      Filter events on server side, in order to decrease load on Marathon (#541) (#547)
      Split out tini build/install

Shortlog from beta-2 (no Docker image) to beta-3:

git shortlog v1.12.0-1.8.3-beta-2..v1.12.0-1.8.3-beta-3
Justin Lee (2):
      Filter events on server side, in order to decrease load on Marathon (#541) (#547)
      Split out tini build/install

Shortlog from v1.11.3 to beta-3:

git shortlog v1.11.3..v1.12.0-1.8.3-beta-3
Justin Lee (4):
      Handle legacy HAPROXY_HEAD (daemon and expose-fd listeners)
      Beta state fix (#538)
      Filter events on server side, in order to decrease load on Marathon (#541) (#547)
      Split out tini build/install

vixns (2):
      Upgrade to haproxy 1.8.2 (#527)
      bump to haproxy v1.8.3 (#536)

Changes to beta since this release: v1.12.0-1.8.3-beta-3...beta