fix: fix wrong scopes for self signed jwt

[arithmetic1728]

Fixes: #929

When auth library creates self signed jwt, it looks for scopes, audience and default_scopes in order, and uses the first one available.

In the transport base class, we are using self.AUTH_SCOPES as scopes if scopes are not provided. This is not right.

gapic-generator-python/gapic/templates/%namespace/%name_%version/%sub/services/%service/transports/base.py.j2

Line 114 in ceb64d9

self._scopes = scopes or self.AUTH_SCOPES

This is not right since self.AUTH_SCOPES should be only used as default_scopes. self.AUTH_SCOPES may contain only a generic "https://www.googleapis.com/auth/cloud-platform" scope, for instance for all the failed APIs listed in this issue. If it is used as scopes in self signed jwt, the samples may fail with invalid scopes. For the success APIs, the AUTH_SCOPES of kms contains an additional https://www.googleapis.com/auth/cloudkms scope so it has no problem; the other two API are mysterious to me since they are using the same generic scope in AUTH_SCOPES as the failed APIs.

The fix is simple. We just need to change self._scopes = scopes or self.AUTH_SCOPES to self._scopes = scopes in the transport base class. Note that self.AUTH_SCOPES is already passed to grpc channel as default_scopes.

gapic-generator-python/gapic/templates/%namespace/%name_%version/%sub/services/%service/transports/grpc.py.j2

Lines 208 to 217 in ceb64d9

	return grpc_helpers.create_channel(
	host,
	credentials=credentials,
	credentials_file=credentials_file,
	quota_project_id=quota_project_id,
	default_scopes=cls.AUTH_SCOPES,
	scopes=scopes,
	default_host=cls.DEFAULT_HOST,
	**kwargs
	)

I tested the PR for all the failed APIs, now all the samples passed (I didn't update the unit test, so kokoro would fail in these PRs):
googleapis/python-speech#189
googleapis/python-asset#215
googleapis/python-videointelligence#171
googleapis/python-analytics-data#95

I also tested the secret_manager and automl APIs, their samples still pass with this fix.
googleapis/python-secret-manager#141
googleapis/python-automl#179