[FEATURE]: exclude tags with 'buildtest buildspec validate'

[shahzebsiddiqui]

Please describe your feature

Let's add a feature --exclude-tags to exclude tags when validating buildspecs. We should have a short option -xt.

TODO

• Add command line option
• Add implementation in https://github.com/buildtesters/buildtest/blob/devel/buildtest/cli/buildspec.py
• Add regression test in https://github.com/buildtesters/buildtest/blob/devel/tests/cli/test_buildspec.py
• Add to bash completion script
• Add documentation in https://buildtest.readthedocs.io/en/devel/gettingstarted/buildspecs_interface.html#validate-buildspecs-buildtest-buildspec-validate

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Additional Information

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Is there an existing issue

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