Add support for elyra-metadata 'create' and 'update' commands

[ptitzler]

Closes #2563.

Follow-up is required in Elyra version 4.0

• to remove support for elyra-metadata install in https://github.com/elyra-ai/elyra/blob/master/elyra/metadata/metadata_app.py.
• to remove the test_install_* unit tests in https://github.com/elyra-ai/elyra/blob/master/elyra/tests/metadata/test_metadata_app.py

What changes were proposed in this pull request?

• Introduce new elyra-metadata create command
• Introduce new elyra-metadata update command
• Mark elyra-metadata install as deprecated
  • this can be observed, for example, by running elyra-metadata -h or elyra-metadata install <schemaspace-name> -h
• Update documentation that references the deprecated elyra-metadata install command to use elyra-metadata create and elyra-metadata update instead
• Add/update existing unit test cases
• Replace elyra-metadata install with elyra-metadata create in all Dockerfiles
• Update front-end tests that were using the elyra-metadata install command

With all changes in place there should be

• no more elyra-metadata install strings in any files in this repository
• no more --replace strings in any files in this repository, with the exception of metadata_app.py and test_metadata_app.py. The related source code will be removed in version 4.0.

The new unit test cases were derived from the existing elyra-metadata install test cases.

How was this pull request tested?

• Added unit test cases for elyra-metadata create
• Added unit test cases for elyra-metadata update
• Reviewed the output of make docs
• Manual testing

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[kevin-bates]

These changes look good - thank you @ptitzler.

Could you please confirm that the create code is a direct copy of the previous install code with update_mode handling instead of --replace handling?

[ptitzler]

These changes look good - thank you @ptitzler.

Could you please confirm that the create code is a direct copy of the previous install code with update_mode handling instead of --replace handling?

Yes, it is! The only changes are in __init__ and start. The other methods are identical.
Because SchemaspaceInstall is removed in 4.0, I didn't want to to through the extra effort to change it's implementation to inherit from SchemaspaceInstall.