Add sql_max_pool_size option

fixes bug 1087055

grizzly adds a new option to nova: sql_max_pool_size, which is
the max number of connections to keep open in a pool. This patch
adds it to the config reference section.

Patch 2 adds sql_max_overflow option and fixes a typo in the
previous commit

Patch 3
fixes bug 1087054
adds sql_min_pool_size and sql_dbpool_enable options

Change-Id: Id9d122e3002c2a91aaf83560f4bbff06911517f3
Merge "Add sql_max_pool_size option"