Replacing direct import for OAuth plugin

The direct import of the auth plugin
'keystone.auth.plugins.oauth1.OAuth' is deprecated since
Liberty.

The entrypoint should only be defined in case there is a
need to override the default implementation with a custom
class.

A closer inspection to the code confirms that:
70c9dd8256/keystone/conf/auth.py (L63)

Closes-Bug: #1837109

Change-Id: Icbad28cdefbccb6e6499ad4e19ad0d6bfaeff677

templates/mitaka/keystone.conf

@@ -63,7 +63,6 @@ driver = {{ assignment_backend }}
 [auth]
 methods = external,password,token,oauth1,totp
 password = keystone.auth.plugins.password.Password
-oauth1 = keystone.auth.plugins.oauth1.OAuth
 
 [paste_deploy]
 config_file = /etc/keystone/keystone-paste.ini

templates/ocata/keystone.conf

@@ -75,7 +75,6 @@ driver = {{ assignment_backend }}
 [auth]
 methods = external,password,token,oauth1,mapped,openid,totp
 password = keystone.auth.plugins.password.Password
-oauth1 = keystone.auth.plugins.oauth1.OAuth
 {% endif %}
 
 [paste_deploy]

templates/queens/keystone.conf

@@ -74,7 +74,6 @@ driver = {{ assignment_backend }}
 [auth]
 methods = {{ auth_methods }}
 password = keystone.auth.plugins.password.Password
-oauth1 = keystone.auth.plugins.oauth1.OAuth
 {% endif %}
 
 [paste_deploy]