etcd: if etcd is not working, acquire() raises an exception

[John Schwarz]

See [1]: in case self.client.put() raises RequestException, and if watch == None, then the code doesn't break - it will proceed to line 123, where UnboundLocalError is raised from the code.

Reproduction steps: stop etcd, try to acquire lock.

[1]: https://github.com/openstack/tooz/blob/5149c402db09d5997b6fc086dbd7d3b1d12815c8/tooz/drivers/etcd.py#L112

[John Schwarz (jschwarz)]

Traceback: http://paste.openstack.org/show/538889/