SolarView: Allow configuration of command

[git-developer]

This PR contains:

• A fix for a bug that was introduced when the original shell script was moved to python.
  • The shell script used the first argument as command, defaulting to 00*.
  • The python module uses the string 1:-00*.
• A new feature to read a custom command from the configuration file, as requested in the forum. This allows users to configure a single inverter as relevant for openWB, and not the whole plant.

The original contribution was #245.

[benderl]

Danke für den PR und Bugfix.
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