Improve first installation playbook to not use serial

[guidograzioli]

When a cluster of keycloaks is installed for the first time, liquibase needs to be run for initializing the database.
Even if a dblock is in the jpa configuration, the nodes of clusters that are started at the same time, concurrently
try to perform the database update, resulting in errors. Current workaround is to have playbook run with

serial:
  - 1
  - 100%

which will run the full playbook on the first host, and then run again on remaining nodes.

Improvement is needed to check database state before starting the service, and in case an update is needed,
only serialize the startup of instances (along with the wait task to let the database update terminate), not the whole play.