[FEATURE]: specify multiple tagnames as comma separated list

[shahzebsiddiqui]

Please describe your feature request

currently if one wants to specify multiple tags one has to do buildtest bd -t TAG1 -t TAG2. Let's keep the append logic for now it would be nice if we can do buildtest bd -t TAG1,TAG2 which is equivalent to two tag names.

We should still be able to preserve append feature so it should be possible to do buildtest bd -t TAG1,TAG2 -t TAG3.

TODO:

• Add implementation
• Add regression test in https://github.com/buildtesters/buildtest/blob/devel/tests/cli/test_build.py
• Add documentation in in https://buildtest.readthedocs.io/en/devel/gettingstarted/buildingtest.html#building-by-tags
• Update buildtest help build

Post question in Slack

• I agree that I posted my question in slack before creating this issue

Is there an existing issue

• I confirm there is no existing issue for this issue