[eventlet-removal] rewrite wait_until_true function

there's no reason to use eventlet for basic 'wait for true' function,
all the tests are running syncronously one by one anyway
(in separate worker processes at most), and merely importing eventlet
can have some unexpected side-effects (for example [0]).

[0] https://storyboard.openstack.org/#!/story/2010902

Change-Id: I9178d9acc8d88e2ddb8fba1111f12a5d2681e33e
Signed-off-by: Pavlo Shchelokovskyy <shchelokovskyy@gmail.com>
3 files changed