port-instrument associations

@edwardhunter please review and merge.

One of the new tests:

    bin/nosetests -sv  --with-pycc ion/agents/platform/test/test_platform_agent_with_rsn.py:TestPlatformAgent.test_turn_on_and_off_port_given_instrument

Note, this is a good progress but the overall port-instrument associations handling is *not* complete yet, as it involves quite a few places also because this is related with long outstanding pending revision, https://jira.oceanobservatories.org/tasks/browse/OOIION-1495