Fix a run condition when creating monitoring metrics
Fix a run condition when creating monitoring metrics

Using JSONLogGroup reference to get the log group name instead of
creating the name string forces cloudformation to wait until the
log group resource has finished creating. Without the dependency
both log group and metric creation are started at the same time
and metric fails since the log group name doesn't exist yet.

alphagov/digitalmarketplace-aws,alphagov/digitalmarketplace-aws,alphagov/digitalmarketplace-aws