Enclose output member/group reference within JSON on add

[tatry]

Closes #20

Affected commands:

• action-selector add-member
• action-selector create-group

PTF tests: https://github.com/tatry/p4c/pull/16

Example output JSON:

• When adding a new member:

  {
    "ingress_as": {
        "added_member_ref": 1
    }
  }

• When adding a new group:

  {
    "ingress_as": {
        "added_group_ref": 1
    }
  }