Adding a `heptapod` attribute on Heptapod

For objects that are relative to different type of GitLab objects,
such as Runners, it would be convenient to access the Heptapod
instance uniformly.
2 jobs for branch/default in 11 minutes and 25 seconds
Name Stage Failure
test-latest Test
ERROR tests/[True]
ERROR tests/[False]
ERROR tests/ - AssertionError: Heptapod serv...
ERROR tests/ - AssertionError: Heptapod server d...
================== 18 skipped, 64 errors in 601.45s (0:10:01) ==================
ERROR: InvocationError for command /builds/heptapod/heptapod-tests/.tox/py3/bin/py.test -v --heptapod-url http://2620:52:3:1:ec4:7aff:fef7:a700 --heptapod-ssh-port 22 --heptapod-remote --heptapod-webdriver-remote-url http://selenium:4444/wd/hub --heptapod-wait-after-first-response 180 (exited with code 1)
___________________________________ summary ____________________________________
ERROR: py3: commands failed
ERROR: Job failed: exit code 1