Enable "wait_until_true" when used ouf the main thread

Enable "wait_until_true" when used ouf the main thread

"util.wait_until_true" uses "util.Timer" to create an active wait for a
condition. "Timer" class uses "signal" to create a watchdog to track the
time lapsed.

When used out of the main thread, "Timer" raises the following error:
  ValueError: signal only works in main thread

To make "util.wait_until_true" usable always, a new waitting method is
implemented.

JIRA: YARDSTICK-1358

Change-Id: Ifb5ba0b17b5beca0af5ceab4f6431d58b7928762
Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>