PM-208: Implement restoration of snapshots in the java-server role.

Context

• Project README link: in the review!
• Task details: https://its.cern.ch/jira/browse/PM-208

What was done

• Added a new task to restore an existing snapshot.
• If a deployment fails, we automatically attempt to restore the latest snapshot.

QA

• Classic Ansible "we have no idea how to test effectively" statement

Review objectives

• assessment of code functionality
• assessment of code quality
• knowledge sharing