avocado.utils: Add wait library

Add a library for functions with the same pattern: wait for
another function to evaluate to True, for a certain amount
of time.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>

avocado/utils/wait.py

+import logging
+import time
+
+log = logging.getLogger('avocado.test')
+
+
+def wait_for(func, timeout, first=0.0, step=1.0, text=None):
+    """
+    Wait until func() evaluates to True.
+
+    If func() evaluates to True before timeout expires, return the
+    value of func(). Otherwise return None.
+
+    :param timeout: Timeout in seconds
+    :param first: Time to sleep before first attempt
+    :param steps: Time to sleep between attempts in seconds
+    :param text: Text to print while waiting, for debug purposes
+    """
+    start_time = time.time()
+    end_time = time.time() + timeout
+
+    time.sleep(first)
+
+    while time.time() < end_time:
+        if text:
+            log.debug("%s (%f secs)", text, (time.time() - start_time))
+
+        output = func()
+        if output:
+            return output
+
+        time.sleep(step)
+
+    return None