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).

(cherry picked from commit 2e27e7528da7c1c9ad22dbec8eccdde92f2b6a9c)