Fix sqlalchemy related quantum.conf options

idle_timeout, min_pool_size, max_pool_size and pool_timeout are not valid
options from quantum.conf. Use the correct ones now. Removed the "idle_timeout"
option as there is no equivalent for it in quantum.conf unless using MySQL with
eventlet's db_pool which our config doesn't. Renamed max_pool_size to
max_pool_overflow to better reflect what it is used for.

Increase the default values for min_pool_size as quantum seems to be greedy
on database connections (see: https://bugs.launchpad.net/tripleo/+bug/1184484)

Use the default value of 30 for pool_timeout, as this is what quantum was using
up to now.