Reset the plugin information when all_plugins is called consecutively in order to pick up any changes to the plugins that happened in the meantime.

This is needed for ohai resource of Chef which calls all_plugins during :reload action.
Fix and/or Turn off the errors, warnings from ohai in the default run condition.
Make sure the file_path is passed to eval.
Remove the forgotten :focus in system tests.
Fix network plugin specs.
Move constant clean up functions to its own mixin.
Decrease the chance of specs failing to 1 / 2^32
Convert @ohai => let(:ohai) in system_spec.
Merge pull request #274 from opscode/OC-10998

OC-10998: Test & Fix Chef ohai resource