Add variables for the following 3 nova.conf vars:
max_overflow (default 10)
max_pool_size (default 5)
pool_timeout (default 30)
This allows for sql tuning to better support bulk operations
(boot/delete) with the ability to define custom values in nova.conf
based on business needs.
Change-Id: Ic427e6822f636a304cbbfaab5ac74a13e912da0f
Closes-Bug: #1447389