adding parameter to configure QueuePool in SQLAlchemy

This patch introduces the parameter sqlalchemy_pool_size.
The parameter is used to configure the QueuePool, created
in configure_db(). At the moment the configuration of the
pool is only possible when using eventlet's db_pool for
MySQL (sql_dbpool_enable = True).

Change-Id: I64dec2863c1ee88215ef9b28054ba3bfaafa3b32
fixes: bug #1158235
Merge "adding parameter to configure QueuePool in SQLAlchemy"