Always add an explicit time zone to `recovery_target_time`

[barthisrael]

Previous to this commit there could be cases where Postgres would be misconfigured by barman recover.

For example, assume you had an environment like this:

• Barman host configured with time zone UTC+2, e.g. Europe/Berlin;`
• Postgres cluster with default value for timezone GUC, i.e. GMT value.

If we attempted a barman recover execution with --target-time set, but without an explicit time zone, recovery could fail:

• barman recover output would say that recovery target time was set with +02:00;
• However postgresql.auto.conf would not contain +02:00.

In practice either the recovery would fail because of an invalid recovery target, or the cluster would be recovered to a point-in-time different from the user needs. Neither of these cases are welcome.

This commit fixes that issue by reusing the target_datetime key from the recovery_info variable. That entry contains the target time with a time zone explicitly set, even if the user specified no time zone through the command-line option.

Note: a warning is issued if the user specified no time zone, and Barman assumed its host time zone.

References: BAR-185 #881.