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)
Merge pull request #391 from claco/sql-pool-settings

Merged automatically by jenkins after successful build test