fixing bugs when some, but not all, custom API URLs are specified

[tomreitz]

@rlittle08 was trying to manually specify a dependencies_url but found a bug, that it doesn't work unless you also specify an oauth_url. This PR fixes that, allowing any of the following to be manually specified to override the defaults discovered from the base_url:

• oauth_url
• dependencies_url
• open_api_metadata_url or both descriptors_swagger_url and resources_swagger_url

A future improvement might be to also allow a custom dataManagementApi URL (instead of constructing it in get_data_url()), and to make the mode, year, and version optional, since those can also be discovered from the base_url.

Note that with these improvements, it is now theoretically possible to validate against a different open_api_metadata_url then you send to (base_url) in a single lightbeam.yaml and even lightbeam validate+send call. I'm not sure if anyone would ever need/want to do that, but it's possible.