OOIION-1172: Stop storing agent process IDs in resources

This patch removes dependence on the agent instance resource for keeping track of the agent process ID.  Instead, this is retrieved directly from the ResourceAgent classmethod.  

All IMS code and tests have had references to this field removed.

SMOKE tests pass as do the following affected tests:
* ion.services.sa.instrument.test.test_instrument_management_service_integration:TestInstrumentManagementServiceIntegration.test_resource_state_save_restore 
* ion.services.sa.test.test_activate_instrument:TestActivateInstrumentIntegration.test_activateInstrumentSample 
* ion.services.sa.test.test_driver_egg:TestDriverEgg.test_driverLaunchBogusModuleWithURI 
* ion.services.sa.test.test_driver_egg:TestDriverEgg.test_driverLaunchModuleWithURI 
* ion.services.sa.test.test_driver_egg:TestDriverEgg.test_driverLaunchNoModuleBadEggURI 
* ion.services.sa.test.test_driver_egg:TestDriverEgg.test_driverLaunchNoModuleOnlyURI 
* ion.services.sa.test.test_instrument_alerts:TestInstrumentAlerts.test_alerts