Expose most common QueuePoll parameters from SQLAlchemy

Fixes bug 1160442

The defualt parameters are used unless otherwise configured. This
is applicable for:
 - pool_size
 - max_overflow
 - pool_timeout

By increasing the pool_size and the max_overflow and lowering the timeout
the issue reported in the bug is resolved.

NOTE: the configuration values enable the user to make use of the default
QueuePool tuning values.

(cherry picked from commit 725d636da5959aee1954a8e8d5ff383d2a8b7bb9)

Change-Id: Ic258442531a2caf2db4f05c4ddd6fbfb0c180f17
Merge pull request #6 from hyunsun/bugfix

Expose most common QueuePoll parameters from SQLAlchemy