Improve DN usage in ipa-client-install

A hotfix pushed in a scope of ticket 3088 forced conversion of DN
object (baseDN) in IPA client discovery so that ipa-client-install
does not crash when creating an IPA default.conf. Since this is not
a preferred way to handle DN objects, improve its usage:

- make sure, that baseDN retrieved by client discovery is always
  a DN object
- update ipachangeconf.py code to handle strings better and instead
  of concatenating objects, make sure they are converted to string
  first

As a side-effect of ipachangeconf changes, default.conf config file
generated by ipa-client-install has no longer empty new line at the
end of a file.

Whole ipachangeconf.py has been modified to be compliant with PEP8.

https://fedorahosted.org/freeipa/ticket/3088

ipapython/ipautil.py

@@ -831,7 +831,7 @@ def get_ipa_basedn(conn):
                 % (info, IPA_BASEDN_INFO))
             continue
         root_logger.debug("Naming context '%s' is a valid IPA context" % context)
-        return context
+        return DN(context)
 
     return None