Remove the check if the output directory exists when starting lorax.

Pungi creates the output directory before invoking the buildinstall,
so we don't want to require that the directory does not exist.

src/pylorax/__init__.py

@@ -78,7 +78,6 @@ class Lorax(BaseLoraxClass):
                                                 updatesdir=updatesdir)
 
         # create the output directory
-        assert not os.path.isdir(outputdir), "output directory exists"
         self.outputdir = outputdir
         makedirs_(self.outputdir)