Allow to backup on any S3-compatible service

Starting from the versions subsequent to the v57a, the default backend
of duplicity for backups to S3-compatible services is boto3
which requires the indication of the region (--S3-region-name)
for most cases in which the S3 service is not provided by AWS.
Thanks to this intervention, the region of the S3 service
is provided as a parameter to duplicity and is stored in the
/home/user-data/backup/custom.yaml file, like any other backup
configuration parameters.
Moreover, the list of the regions shown to the user in the drop-down
menu in the backup configuration panel is replaced with
a normal text field, since the list included the AWS regions only
and therefore was not usable in all other cases.

See issue #2200.

api/mailinabox.yml

@@ -468,6 +468,7 @@ paths:
                   target: s3://s3.eu-central-1.amazonaws.com/box-example-com
                   target_user: ACCESS_KEY
                   target_pass: SECRET_ACCESS_KEY
+                  target_region: eu-central-1
                   minAge: 3
               local:
                 summary: Local backup
@@ -475,6 +476,7 @@ paths:
                   target: local
                   target_user: ''
                   target_pass: ''
+                  target_region: ''
                   minAge: 3
               rsync:
                 summary: Rsync backup
@@ -482,6 +484,7 @@ paths:
                   target: rsync://username@box.example.com//backups/box.example.com
                   target_user: ''
                   target_pass: ''
+                  target_region: ''
                   minAge: 3
               off:
                 summary: Disable backups
@@ -489,6 +492,7 @@ paths:
                   target: 'off'
                   target_user: ''
                   target_pass: ''
+                  target_region: ''
                   minAge: 0
       x-codeSamples:
         - lang: curl
@@ -497,6 +501,7 @@ paths:
               -d "target=<hostname>" \
               -d "target_user=<string>" \
               -d "target_pass=<password>" \
+              -d "target_region=<region>" \
               -d "min_age=<integer>" \
               -u "<email>:<password>"
       responses:
@@ -2460,6 +2465,7 @@ components:
         - target
         - target_user
         - target_pass
+        - target_region
         - min_age
       properties:
         target:
@@ -2473,6 +2479,9 @@ components:
           type: string
           example: password
           format: password
+        target_region:
+          type: string
+          example: eu-central-1
         min_age:
           type: integer
           format: int32
@@ -2514,6 +2523,9 @@ components:
           type: string
         target_pass:
           type: string
+        target_region:
+          type: string
+          example: eu-central-1
       description: Backup config response.
     SystemBackupStatusResponse:
       type: object