BUG/MINOR: server: remove x-nullable

Removed x-nullable from on-marked-down, on-marked-up and on-error
options in server.

Author: mjuraga

build/haproxy_spec.yaml

@@ -759,17 +759,14 @@ definitions:
           - sudden-death
           - mark-down
           type: string
-          x-nullable: true
         on-marked-down:
           enum:
           - shutdown-sessions
           type: string
-          x-nullable: true
         on-marked-up:
           enum:
           - shutdown-backup-sessions
           type: string
-          x-nullable: true
         port:
           maximum: 65535
           minimum: 0

models/haproxy.yaml

@@ -515,15 +515,12 @@ server:
     on-error:
       type: string
       enum: [fastinter, fail-check, sudden-death, mark-down]
-      x-nullable: true
     on-marked-down:
       type: string
       enum: [shutdown-sessions]
-      x-nullable: true
     on-marked-up:
       type: string
       enum: [shutdown-backup-sessions]
-      x-nullable: true
     ssl:
       type: string
       enum: [enabled, disabled]