Fixes a bug with connecting to the agent

[elgris]

Basically, it fixes 2 pieces:

1. Hanging shell call in Docker container (as discovered, we need a shell=True there). Also the command itself was taken from golang sensor
2. One small bug when value must be updated before the condition

[pglombardo]

A debug log issue surfaced in python 3.6.2. I'll fix in a separate PR before release:
https://travis-ci.org/instana/python-sensor/jobs/275808812

[elgris]

thanks @pglombardo !
For me changing a line fsm.py:86 from

proc.stdout.read()

to

str(proc.stdout.read())

fixed Python3 tests

[pglombardo]

That might be the more appropriate place. I did the same just in the actual debug line in check_host with str().