default: fix case where 'location' does not contain any directory separator

This fixes:

  Traceback (most recent call last):
    ...
    File "slapos/recipe/build/__init__.py", line 268, in install
      return self._shared.install(self.__install)
    File "slapos/recipe/__init__.py", line 149, in install
      os.makedirs(parent)
    File "/usr/lib/python2.7/os.py", line 157, in makedirs
      mkdir(name, mode)
  OSError: [Errno 2] No such file or directory: ''

slapos/recipe/__init__.py

@@ -145,7 +145,7 @@ class Shared(object):
         os.makedirs(location)
       else:
         parent = os.path.dirname(location)
-        if not os.path.isdir(parent):
+        if parent and not os.path.isdir(parent):
           os.makedirs(parent)
       install()
       try: