db.db_create() shouldn't call hiddenly db.db_drop()

It is nasty things that could produce a lot of problems.

Change-Id: I8a04c70817a9b975d6064693a2d27597c4b54d3e

rally/db/sqlalchemy/models.py

@@ -53,7 +53,6 @@ class Task(BASE, RallyBase):
 
 
 def create_db():
-    drop_db()
     BASE.metadata.create_all(session.get_engine())
 
 

rally/test.py

@@ -27,6 +27,7 @@ class DatabaseFixture(config.Config):
         super(DatabaseFixture, self).setUp()
         db.db_cleanup()
         self.conf.set_default('connection', "sqlite://", group='database')
+        db.db_drop()
         db.db_create()