Refactor Barman scripts and add a sub-command to switch Barman config (#3016)

We currently have a script named `patroni_barman_recover` in Patroni, which is intended to be used as a custom bootstrap method, or as a custom replica creation method.

Now there is need of one more Barman related script in Patroni to handle switching of config models in Barman upon `on_role_change` events.

However, instead of creating another Patroni script, let's say `patroni_barman_config_switch`, and duplicating a lot of logic in the code, we decided to refactor the code so:

* Instead of two separate scripts (`patroni_barman_recover` and `patroni_barman_config_switch`), we have a single script (`patroni_barman`) with 2 sub-commands (`recover` and `config-switch`)

This is the overview of changes that have been performed:

* File `patroni.scripts.barman_recover` has been removed, and its logic has been split into a few files:
  * `patroni.scripts.barman.cli`: handles the entrypoint of the new `patroni_barman` command, exposing the argument parser and calling the appropriate functions depending on the sub-command
  * `patroni.scripts.barman.utils`: implements utilitary enums, functions and classes wich can be used by `cli` and by sub-commands implementation:
    * retry mechanism
    * logging set up
    * communication with pg-backup-api
  * `patroni.scripts.barman.recover`: implements the `recover` sub-command only
* File `patroni.tests.test_barman_recover` has been renamed as `patroni.tests.test_barman`
* File `patroni.scripts.barman.config_switch` was created to implement the `config-switch` sub-command only
* `setup.py` has been changed so it generates a `patroni_barman` application instead of `patroni_barman_recover`
* Docs and unit tests were updated accordingly

References: PAT-154.

setup.py

@@ -55,7 +55,7 @@ CONSOLE_SCRIPTS = ['patroni = patroni.__main__:main',
                    'patroni_raft_controller = patroni.raft_controller:main',
                    "patroni_wale_restore = patroni.scripts.wale_restore:main",
                    "patroni_aws = patroni.scripts.aws:main",
-                   "patroni_barman_recover = patroni.scripts.barman_recover:main"]
+                   "patroni_barman = patroni.scripts.barman.cli:main"]
 
 
 class _Command(Command):