cloudapi/v2: tweak customizations

Fix verbiage of groups customization, fields which accept an array
should be plural.

Remove the sshkey customization, sshkeys are merged into user
customizations anyway, so users should use the "users" customization
instead.

Since these customizations aren't in use yet, this edit should be fine.

See #3716

internal/cloudapi/v2/openapi.v2.yml

@@ -1201,12 +1201,7 @@ components:
           example: myhostname
         kernel:
           $ref: '#/components/schemas/Kernel'
-        sshkey:
-          type: array
-          description: List of ssh keys
-          items:
-            $ref: '#/components/schemas/SSHKey'
-        group:
+        groups:
           type: array
           description: List of groups to create
           items:
@@ -1450,22 +1445,6 @@ components:
           type: string
           description: Appends arguments to the bootloader kernel command line
           example: nosmt=force
-    SSHKey:
-      type: object
-      additionalProperties: false
-      required:
-      - user
-      - key
-      properties:
-        user:
-          type: string
-          description: User to configure the ssh key for
-          example: admin
-        key:
-          type: string
-          description: Adds the key to the user's authorized_keys file
-          example: |
-            ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIASWitkOH4U874EdsBUnytb3bwvRggHlQlbYXl7n10v9
     Services:
       type: object
       additionalProperties: false