[FEATURE REQUEST ] Add update_service functionality to modules.swarm

Description of Issue

The docker python api supports updating services (similar to docker service update): https://docker-py.readthedocs.io/en/stable/services.html#docker.models.services.Service.update . It would be helpful to have this functionality in the salt 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 by reading the modules.swarm entry point and the Docker Python API Service.update documentation linked in the issue. The work is done when the Salt module provides update_service functionality corresponding to Docker service update behavior.