Add postgresql role

[rpignolet]

The role is designed to deploy a PostgreSQL server. Client configuration is missing and will be added in future PR when needed.

Which issue(s) this PR fixes:

Fixes #6

Additional comments:

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[kpgtek]

By default, server logs are stored in data_directory/log, it is possible to move them to /var/log/postgresql, by setting log_directory = '/var/log/postgresql' in postgresql.conf and creating the directory.

[rpignolet]

Last force push resolve README conflicts.