Add docker engine collector

[ChristianKniep]

Collector to fetch /metrics endpoint of experimental metrics-addr feature.

rootdir: ./src/github.com/ChristianKniep/tcollector, inifile:
plugins: cov-2.4.0
collected 4 items

collectors/test/test_docker_engine.py ....

---------- coverage: platform darwin, python 2.7.12-final-0 ----------
Name                            Stmts   Miss  Cover
---------------------------------------------------
collectors/0/docker_engine.py      68     18    74%

Example output:

$ python collectors/0/docker_engine.py
docker.engine_daemon_container_actions_seconds_bucket 1481211084 1.0 action=changes le=0.005
docker.engine_daemon_container_actions_seconds_bucket 1481211084 1.0 action=changes le=0.01
*snip*
docker.engine_daemon_engine_cpus_cpus 1481211084 2.0
docker.engine_daemon_engine_info 1481211084 1.0 architecture=x86_64 commit=88a45c4 graph_driver=aufs kernel=4.4.0-38-generic os=Ubuntu 16.04.1 LTS version=1.13.0-rc1
docker.engine_daemon_engine_memory_bytes 1481211084 4143755264.0
docker.engine_daemon_events_subscribers_total 1481211084 0.0
docker.engine_daemon_events_total 1481211084 8.0
docker.engine_daemon_health_checks_failed_total 1481211084 0.0
docker.engine_daemon_health_checks_total 1481211084 0.0
*snip*

[johann8384]

Merged in #386