network: Remove outdated servers

Previously it could happen that servers which were removed in the TUI
didn't get removed in teh actual config files.
This is now fixed by removing all existing servers and writing the new
ones.

Change-Id: I412f046b603c9fe0a3d6768f1a8ced3dc263b07e
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=982534
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>