Fixes MariaDB to support Heat

When spawning Heat stacks, the DB reaches the max_connections
limit. This causes MariaDB to block all traffic by source IP,
essentially blocking all services. This patch introduces the
MARIADB_MAX_CONNECTIONS parameter to make the max_connections
configurable. The default of 151 max_connections is maintained.

Closes-Bug: #1465422

Change-Id: I869aa9f117c6fa959b1c6948dfc379f30a6bc1d7