Preventing __builtins__ from being in HORIZON_CONFIG

When we use the 'enabled' mechanism to add panels,
__builtins__ is also getting added to HORIZON_CONFIG.
Since that is undesirable, it is now prevented
by stripping it out before it gets added.

Change-Id: I3dc772210de8da9810795368df9bf1123388624d
Closes-Bug:  bug 1312180
Merge "Preventing __builtins__ from being in HORIZON_CONFIG"