Accept tokens from env variables

[mohabusama]

This can be useful when testing against staging environment with tokens not supported by zign

[codecov[bot]]

Codecov Report

Merging #64 into master will decrease coverage by 0.04%. The diff coverage is 50%.

@@            Coverage Diff             @@
##           master      #64      +/-   ##
==========================================
- Coverage   74.13%   74.08%   -0.05%     
==========================================
  Files          18       18              
  Lines        1040     1042       +2     
==========================================
+ Hits          771      772       +1     
- Misses        269      270       +1

Impacted Files	Coverage Δ
zmon_cli/cmds/command.py	89.09% <50%> (-1.48%)	:arrow_down:

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[elgris]

👍

[mohabusama]

👍