Merge mysql service_config_settings for nova

Merge mysql service_config_settings for nova

The hieradata from the nova-base.yaml service template needs to be available to
the role running mysql, which isn't necessarily the same role as nova.

nova-base.yaml isn't an actual service template though that is included in any
ServiceChain resources, it's outputs need to be merged with an actual nova
service template, such as nova-api.yaml.

As nova-api.yaml already provides some hieradata for the mysql service in
service_config_settings, this patch uses map_merge to combine the 2 values.

Change-Id: I4dc684b3611b13f177f9499e69468d3f6ef6fa76
Closes-Bug: #1654058