PEP-8 fixes for a few Tool Shed install components.
 - lib/tool_shed/galaxy_install/install_manager.py
 - lib/tool_shed/galaxy_install/repository_dependencies/repository_dependency_manager.py
 - lib/tool_shed/galaxy_install/repair_repository_manager.py
Tweak two ToolShed modules to clarify they don't use ToolSection...
... objects (directly anyway). I would like to refactor all the tool shed components to not deal directly with ToolSection objects but instead just pass around IDs.
More centralization of code for registering tools in ToolBox.
Eliminate one reference to ToolBox.tool_panel in install_manager.py.
Simplify shed-y logic in load_tool_tag_set.
Drops cycolomatic complexity from 14 to 10 for that method.
De-duplication for loading elements into the tool panel.
Create one public method load_item on ToolBox that should be used externally instead of load_tool_tag_set, load_section_tag_set, etc.... This reduces the duplication within the toolbox and between the toolbox and the Tool Shed's ToolPanelManager. Probably more importantly it also is another step down the road toward hiding implementation details such as toolbox.tool_panel and toolbox.intergrated_tool_panel from the ToolPanelManager.

Already had unit test coverage for ToolPanelManager stuff here - but added test coverage for loading labels, workflows, and tool directories.