Do not fail immediately if wakeup fails, retry instead

Since we restart keystone immediately and then do a wakeup call,
sometimes the wakeup call fails because it cannot connect to the server
yet.

To avoid that, we should just retry the wakeup.

https://bugzilla.novell.com/show_bug.cgi?id=859678
Merge pull request #162 from vuntz/retry-wakeup

Do not fail immediately if wakeup fails, retry instead