Fix "slapgrid: Map instances to their installed python"

Allow to have missing bin/buildout of the software release, especially to cover
the case, when SR is not installed for partition being processed, eg in order
to destroy it.

slapos/grid/utils.py

@@ -172,7 +172,7 @@ def getPythonExecutableFromSoftwarePath(software_path):
   try:
     with open(os.path.join(software_path, 'bin', 'buildout')) as f:
       shebang = f.readline()
-  except OSError:
+  except (IOError, OSError):
     return
   if shebang.startswith('#!'):
     return shebang[2:].split(None, 1)[0]