Search environment for crossbar

[kjeldflarup]

Uses the environment variable LG_CROSSBAR and LG_CROSSBAR_REALM as defaults for crossbar URL, if they are set.

[codecov-io]

Codecov Report

Merging #209 into master will increase coverage by <.1%.
The diff coverage is 83.3%.

@@           Coverage Diff            @@
##           master    #209     +/-   ##
========================================
+ Coverage    52.6%   52.6%   +<.1%     
========================================
  Files          92      92             
  Lines        5213    5215      +2     
========================================
+ Hits         2745    2747      +2     
  Misses       2468    2468

Impacted Files	Coverage Δ
labgrid/remote/client.py	49.1% <100%> (ø)	⬆️
labgrid/resource/remote.py	84.4% <100%> (+0.1%)	⬆️
labgrid/remote/exporter.py	35.2% <50%> (+0.2%)	⬆️

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[jluebbe]

I've updated your PR with some documentation and reworded the commit message a bit.
Also, the Signed-off-by line is missing, see: https://labgrid.readthedocs.io/en/latest/development.html#developer-s-certificate-of-origin

[kjeldflarup]

I'm not really sure how to fix the missing sign-off, the time --amend trick seems to be passed as I merged upstream before pushing to githup.

[jluebbe]

If you are OK with it, I can also add "Signed-off-by: Kjeld Flarup kfa@deif.com" myself by rebasing it. Otherwise you can add it yourself by updating, editing the message using "git rebase -i" and force-pushing.

[kjeldflarup]

It seems that I succeded!