Rename [DEFAULT] keystone.conf module to keystone.conf.default

The renames the config module which default the [DEFAULT] configuration
section from 'base' (a name adopted based on nova's precedence with
nova.conf.base) to 'default', which should make it slightly easier for
developers to navigate the keystone.conf package, since all other
configuration modules directly reflect the name of the section they're
defining.

Also note, that we could take this one step further and call the module
keystone.conf.DEFAULT, but it's actually a PEP8 violation to name
module in uppercase (it reads like a reference to a constant in the
keystone.conf module). From PEP8:

    Modules should have short, all-lowercase names. Underscores can be
    used in the module name if it improves readability."

Source: https://www.python.org/dev/peps/pep-0008/#package-and-module-names

Change-Id: I78d4ecf67ddcb5c3c46ce0c85e1f8330fbf99877

keystone/conf/__init__.py

@@ -21,9 +21,9 @@ from osprofiler import opts as profiler
 
 from keystone.conf import assignment
 from keystone.conf import auth
-from keystone.conf import base
 from keystone.conf import catalog
 from keystone.conf import credential
+from keystone.conf import default
 from keystone.conf import domain_config
 from keystone.conf import endpoint_filter
 from keystone.conf import endpoint_policy
@@ -57,9 +57,9 @@ CONF = cfg.CONF
 conf_modules = [
     assignment,
     auth,
-    base,
     catalog,
     credential,
+    default,
     domain_config,
     endpoint_filter,
     endpoint_policy,