fixup! Require private key to have a matching certificate

This fixes the following error on Python 3:

  TypeError: startswith first arg must be bytes or a tuple of bytes, not str

slapos/crypto.py

@@ -16,7 +16,7 @@ def dump_publickey(type, pkey):
   assert type == FILETYPE_PEM, type
   pkey.seek(0, 0)
   r = pkey.read()
-  if not r.startswith('-----BEGIN PUBLIC KEY-----'):
+  if not r.startswith(b'-----BEGIN PUBLIC KEY-----'):
     p = Popen(("openssl", "rsa", "-in", pkey.name, "-pubout"),
               stdout=PIPE, stderr=PIPE)
     r, err = p.communicate()