Improve flow when encounter auth failure for GCE service account with no
configured storage scopes:

1. Changed HasConfiguredCredentials() logic not to include has_auth_plugins
as part of the evaluated expression, since that will always evaluate to
true under GCE (since GCE configures its internal service account plugin
under /etc/boto.cfg).

2. Changed ConfigureNoOpAuthIfNeeded logic so we configure no-op auth plugin
even if there is a config_file list, since GCE always configures /etc/boto.cfg,
even if user has no storage scopes configured.

3. Additional changes:

a. Removed assertion of oauth access token cache check log from
test_Doption.py, which may not be true sometimes (e.g., if user is using
HMAC creds).

b. Removed remnants of CONFIG_REQUIRED left over from earlier CL.

c. Merged dupe _ConfigureNoOpAuthIfNeeded functions from two code files,
moved to util.py.