Fix python monasca-api mysql server connection loss

This patch is a fix for MySQL connection timeouts that ocasionally
occurs on the python monasca-api after not being used for about 8
hours.

Reference for the change:
  http://docs.sqlalchemy.org/en/rel_0_9/core/pooling.html#pool-setting-recycle

Change-Id: I8186718aead27f7966e3c35dbd0f490884419955

monasca_api/common/repositories/sqla/sql_repository.py

@@ -50,7 +50,7 @@ class SQLRepository(object):
                     url = URL(**database_conf)
 
             from sqlalchemy import create_engine
-            self._db_engine = create_engine(url)
+            self._db_engine = create_engine(url, pool_recycle=3600)
 
             self.metadata = MetaData()