Added some comments to the enterprise plugin loading code.
Add -no-extensions option to disable plugins during upgrade procedure.

It still needs to be passed to the agent by the upgrade policy code.
Add -no-extensions (-E) option to upgrade policy.

This is to make sure that the upgrade agent (cf-twin) does not try to
load the plugin, since the plugin may be part of the upgrade.
Make enterprise extension test passable on Community-only builds.
Merge pull request #913 from kacfengine/upgrade_path_3_6

Upgrade path 3 6