improve metadata filtering in submit-tuxbuild

[jscook2345]

@chaws had a few good suggestions for better handling metadata filtering in the submit-tuxbuild command.

We need filtering on the data since some of the data in build.json should not be included in squad for various reasons, such as privacy, redundancy, etc.

Suggestion: Use the important_metadata_keys from squad instead of hard-coding them.

ALLOWED_METADATA = Squad().group(args.group).project(args.project).important_metadata_keys

Suggestion: Add an option (with a good default) to filter metadata

--filter-metadata-out=key1,key2