slaprunner: do not use enums of integers in resilient-input-schema.json

Enum values are interpreted as strings in the web ui, which makes
impossible to chosse the number of backups.

Also, set the minimum number of backup to 0, as it is now possible.

software/slaprunner/instance-runner-resilient-input-schema.json

@@ -12,7 +12,8 @@
                     "description": "Amount of backup(s) to create. Each backup consists of a Pull Backup Server and a clone.",
                     "type": "integer",
                     "default": 1,
-                    "enum": [1, 2],
+                    "minimum": 0,
+                    "maximum": 2,
                     "optional": true
                 },
                 "-sla-runner0-computer_guid": {