Issue #665: Attributes for sql pool settings

Expose some Nova SQL attributes:

 - `connection_debug=0`
 - `idle_timeout=3600`
 - `max_pool_size=None`
 - `max_overflow=None`
 - `max_retries=10`
 - `min_pool_size=1`
 - `retry_interval=10`

NB!: These options are different in Grizzly (DO NOT MERGE TO `v4.1.X`).