Move :disabled_plugin check for plugins to running time of the plugins

Currently we check the :disabled_plugins in Ohai::Runner#run_plugin. Dependency resolution for V7 plugins happen after this check. Because of this you can't disable a plugin using its name if that plugin is dependent on by other plugins.

Move the logic to check if a plugin is disabled to the time when we are actually running the plugin so that one can disable a plugin for sure.

/cc: @opscode/client-eng