Change Gerrit MySQL connection option

MySQL does not really support connection pooling over JDBC. Turning
off the connection pool option should make Gerrit less likely to run
into the error : com.mysql.jdbc.exceptions.jdbc4.CommunicationsException

See https://groups.google.com/forum/#!topic/repo-discuss/ranb6VwsMOs and
https://gerrit-review.googlesource.com/#/c/53966/ for details.

Change-Id: I854024ac772bacb6872566c8b05e7540e15fb2c1
Closes-Bug: 1356980