Add index on valid column of the SQL token Backend

Add migration script to add the index

Change-Id: I0d7e231b32040dfb57c2e8023a7f45d65416763c
Fixes: bug #1168399

keystone/common/sql/migrate_repo/versions/025_add_index_to_valid.py

@@ -0,0 +1,17 @@
+import sqlalchemy as sql
+
+
+def upgrade(migrate_engine):
+    meta = sql.MetaData()
+    meta.bind = migrate_engine
+    token = sql.Table('token', meta, autoload=True)
+    idx = sql.Index('ix_token_valid', token.c.valid)
+    idx.create(migrate_engine)
+
+
+def downgrade(migrate_engine):
+    meta = sql.MetaData()
+    meta.bind = migrate_engine
+    token = sql.Table('token', meta, autoload=True)
+    idx = sql.Index('ix_token_valid', token.c.valid)
+    idx.drop(migrate_engine)