Nitpicking
Fix runzeo_logrotate_on_sigusr2

Attempting to open a file before it is created turns out not to be a
good idea.  Moved all the file opening inside the wait_until lambdas
(after extracting them into a standalone helper function, since you
can't use the with statement inside a lambda).

Also fixed test failure on Python 3.3 (r.write() returns the number of
bytes written).

Added an XXX about a possible hang while opening ClientStorage, in case
something went wrong.  I'm not sure how to fix that.
Enable Python 3.x tests in Travis-CI