Set up correct AuthservID defaults

Without this fix, a verifying dkimpy-milter that has no explicit
AuthservID produces the following crashing behavior as it tries to
create the authres header:

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/Milter/__init__.py", line 772, in <lambda>
    milter.set_eom_callback(lambda ctx: ctx.getpriv().eom())
  File "…/dkimpy_milter/__init__.py", line 199, in eom
    h = fold(str(h))
  File "/usr/lib/python2.7/dist-packages/authres/core.py", line 476, in __str__
    return ''.join((self.HEADER_FIELD_NAME, ': ', self.header_value()))
  File "/usr/lib/python2.7/dist-packages/authres/core.py", line 496, in header_value
    return ''.join(strs)

dkimpy_milter/config.py

@@ -401,7 +401,7 @@ def _readConfigFile(path, configData=None, configGlobal={}):
             configData[name] = conversion(value)
     fp.close()
     try:
-        configData['AuthservID'] = _make_authserv_id(configData['AuthservID'])
+        configData['AuthservID'] = _make_authserv_id(configData.get('AuthservID', 'HOSTNAME'))
         configData['IntHosts'] = HostsDataset(configData['InternalHosts'])
     except:
         pass