Support filtering by tag

[musiKk]

When filtering alerts or checks via zmon alert-definitions filter some-field some-value only equality checks are performed. This makes it impossible to filter by tag as tags are a list. I guess usability-wise the least intrusive way of incorporating this feature would be to just change the logic for the tags to check if any tag matches the tag from the command line.

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Initially I wanted to propose lots of other filter mechanisms but then I found out that the zmon Rest API only supports fetching all resources and filtering is done on the client-side anyway so piping the output of zmon *-definitions list --output json to jq is no worse performance-wise. Still, being able to have multiple filter, filtering case-insensitively and having things like "contains" for searching alert descriptions directly from the zmon CLI could be interesting.