Moved the constants that set how often we check for the external tool status and how long a checking task should run before dying in settings.py.  These values can be overwritten in settings_local.py.
moved TOOL_NAME_CHOICES back into core/models.py
Moved the constants back to the ExternalToolStatus model and added in a couple of dictionaries (INTERVAL_BETWEEN_CHECKS and TIMEOUT) for overwriting the default values in the ExternalToolStatus model.
changed task for checking galaxy so that it searches for robots.txt instead of needing to make a database call
Added a field to the model error_logged to prevent our error log from getting thrashed if an external tool is down for a while.
Merge branch 'develop' of github.com:parklab/refinery-platform into node_relationship_ui_feature
Merge migrations of NodeSet model from node_relationship_ui_feature branch and ExternalToolStatus model from develop branch.