fleetctl: allow disabling packs via apply

[nyanshak]

• Fixes issue #2240

[nyanshak]

fleetctl get packs
+--------------------+----------+-------------+----------+
|        NAME        | PLATFORM | DESCRIPTION | DISABLED |
+--------------------+----------+-------------+----------+
| example-pack       |          |             | true     |
+--------------------+----------+-------------+----------+

fleetctl get packs --yaml
---
kind: pack
spec:
  disabled: false
  id: 20
  name: example-pack
  queries:
    - description: ""
      interval: 30
      name: example-query
      query: example-query
  targets:
    labels:
      - All Hosts

fleetctl get pack osquery-monitoring --with-queries
# same output as previous command with the queries:
...
---
apiVersion: v1
kind: query
spec:
  description: example query
  name: example-query
  query: SELECT build_platform FROM osquery_info;