Only create the signature file when it is required.

We should only create the signature files when it is required, to avoid
to have unecessary failures if someone run this test with old version of
slapos.libnetworkcache.

src/zc/buildout/networkcache.txt

@@ -3,8 +3,6 @@ Using buildout with cache over network
 
    >>> nc_data = tmpdir('nc_data')
    >>> nc_url = start_nc(nc_data)
-   >>> signature_data = tmpdir('signature_data')
-   >>> public_certificate_path, private_key_path = create_signature_file_list(signature_data)
 
 Networkcache in buildout is optional, and during normal run no information
 is shown:
@@ -202,6 +200,11 @@ Now lets clean up buildout directory:
    [None, None]
    >>> remove('.installed.cfg')
 
+Creating the signature files:
+
+   >>> signature_data = tmpdir('signature_data')
+   >>> public_certificate_path, private_key_path = create_signature_file_list(signature_data)
+
 Enable signature, so it should not download any content which is not trusted :
    >>> write(sample_buildout, 'buildout.cfg',
    ... '''