wicked CLI should support fetching the correct Helm chart

[DonMartin76]

It would be nice if the wicked-cli would support fetching the correct Helm chart version to a given tag.

Proposed syntax:

wicked helm fetch [-t, --tag <tag>] [-o, --output <dir>] [--no-unpack] [-d, --delete]

The tag would specify the version of the Helm chart to fetch, and the dir the output directory into which to put the unpacked Helm chart to. Specify --no-unpack to not unpack the chart after downloading (keep the .tgz file). Specify -d to delete the output directory's wicked sub directory prior to downloading (or the .tgz file in case --no-unpack was specified).

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This can be implemented without needing to release wicked.