Make handling of schema-name more friendly in CLI

[kevin-bates]

For single-schema namespaces, the CLI tool will default the schema name so it doesn't need to be set. For multi-schema namespaces, the available options will be presented if no schema-name is provided or an invalid schema-name is used. Note: schema-name only comes into play when installing metadata. It is not an option for 'list' or 'remove' subcommands.

Fixes #836

Here are some "screen scrapes"...

$ elyra-metadata install runtime-images
'--name' is a required parameter.

Install a metadata instance into namespace 'runtime-images'.

Options
-------

--replace
	Replace existing instance
--schema_name=<string>
	The schema_name of the metadata instance to install (defaults to 'runtime-image')
--name=<string>
	The name of the metadata instance to install
...

Since no current namespaces have multiple schemas, I've added another test schema to the metadata-tests namespace:

$ elyra-metadata install metadata-tests
'--schema_name' is a required parameter and must be one of the following values: ['metadata-test2', 'metadata-test'].

Install a metadata instance into namespace 'metadata-tests'.

Options
-------

--replace
	Replace existing instance
--schema_name=<string>
	The schema_name of the metadata instance to install.  Must be one of: ['metadata-test2', 'metadata-test']
--name=<string>
	The name of the metadata instance to install

and with an invalid schema name...

$ elyra-metadata install metadata-tests -schema_name=invalid-name
'--schema_name' is a required parameter and must be one of the following values: ['metadata-test2', 'metadata-test'].

Install a metadata instance into namespace 'metadata-tests'.

Options
-------

--replace
	Replace existing instance
--schema_name=<string>
	The schema_name of the metadata instance to install.  Must be one of: ['metadata-test2', 'metadata-test']
--name=<string>
	The name of the metadata instance to install

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