[FEATURE REQUEST] Allow updating labels in salt.modules.swarm.update_node

Description of Issue

The docker node update function exposes a -add-label option. The docker python sdk also seems to have this option in the Node.update() function: https://docker-py.readthedocs.io/en/stable/nodes.html#docker.models.nodes.Node. It would be a nice feature to expose this in the swarm module as well

Versions Report

Master:

Salt Version:
           Salt: 2019.2.2

Dependency Versions:
           cffi: 1.12.3
       cherrypy: Not Installed
      dateutil: 2.7.3
      docker-py: Not Installed
          gitdb: 2.0.5
      gitpython: 2.1.11
          ioflo: Not Installed
         Jinja2: 2.9.4
        libgit2: 0.26.8
        libnacl: Not Installed
       M2Crypto: Not Installed
           Mako: Not Installed
   msgpack-pure: Not Installed
 msgpack-python: 0.5.6
   mysql-python: Not Installed
      pycparser: 2.17
       pycrypto: 2.6.1
   pycryptodome: Not Installed
         pygit2: 0.26.4
         Python: 2.7.16 (default, Apr  6 2019, 01:42:57)
   python-gnupg: Not Installed
         PyYAML: 3.13
          PyZMQ: 17.1.2
           RAET: Not Installed
          smmap: 2.0.5
        timelib: Not Installed
        Tornado: 5.1.1
            ZMQ: 4.3.1

System Versions:
           dist: debian 10.0
         locale: UTF-8
        machine: x86_64
        release: 4.19.0-5-amd64
         system: Linux
        version: debian 10.0

Minion:

Salt Version:
           Salt: 2019.2.2

Dependency Versions:
           cffi: Not Installed
       cherrypy: Not Installed

     dateutil: 2.5.3
      docker-py: 4.1.0
          gitdb: Not Installed
      gitpython: Not Installed
          ioflo: Not Installed
         Jinja2: 2.9.4
        libgit2: Not Installed
        libnacl: Not Installed
       M2Crypto: Not Installed
           Mako: Not Installed
   msgpack-pure: Not Installed
 msgpack-python: 0.4.8
   mysql-python: 1.3.7
      pycparser: Not Installed
       pycrypto: 2.6.1
   pycryptodome: Not Installed
         pygit2: Not Installed
         Python: 2.7.13 (default, Sep 26 2018, 18:42:22)
   python-gnupg: Not Installed
         PyYAML: 3.12
          PyZMQ: 16.0.2
           RAET: Not Installed
          smmap: Not Installed
        timelib: Not Installed
        Tornado: 4.4.3
            ZMQ: 4.2.1

System Versions:
           dist: debian 9.9
         locale: UTF-8
        machine: x86_64
        release: 4.9.0-9-amd64
         system: Linux
        version: debian 9.9

Start at the salt.modules.swarm.update_node entry point and review how it calls the Docker Python SDK's Node.update() method. Add support for updating node labels through the swarm module, then verify that the module exposes Docker's add-label capability and that existing node-update behavior remains intact.