Lazy load oauthlib for plugin loading

If oauthlib is not available the oauth plugins should not be marked as
available when iterating plugins.

Change-Id: Idabb7adf61dc5ca36fc1524af47e346f8f296245

keystoneauth1/extras/oauth1/_loading.py

@@ -22,6 +22,10 @@ class V3OAuth1(loading.BaseIdentityLoader):
     def plugin_class(self):
         return v3.OAuth1
 
+    @property
+    def available(self):
+        return v3.oauth1 is not None
+
     def get_options(self):
         options = super(V3OAuth1, self).get_options()
 

keystoneauth1/extras/oauth1/v3.py

@@ -23,7 +23,10 @@
 
 import logging
 
-from oauthlib import oauth1
+try:
+    from oauthlib import oauth1
+except ImportError:
+    oauth1 = None
 
 from keystoneauth1.identity import v3