Make random_bytes() enforce sane size limits

This makes random_bytes() refuse to generate more than 1MiB of data
at a time. This should almost never be necessary, and we have chased
actual gate OOM failures related to sloppy handling of even
moderately-sized test buffers.

Change-Id: I9cebe778a9309a0f4f5d78be97d062864e1849f1

releasenotes/notes/random-bytes-size-limit-ee94a8c6534fe916.yaml

@@ -0,0 +1,9 @@
+---
+upgrade:
+  - |
+    The ``tempest.lib.common.utils.data_utils.random_bytes()`` helper
+    function will no longer allow a ``size`` of more than 1MiB. Tests
+    generally do not need to generate and use large payloads for
+    feature verification and it is easy to lose track of and duplicate
+    large buffers. The sum total of such errors can become problematic
+    in paralllelized and constrained CI environments.

tempest/lib/common/utils/data_utils.py

@@ -169,6 +169,8 @@ def random_bytes(size=1024):
     :return: size randomly bytes
     :rtype: string
     """
+    if size > 1 << 20:
+        raise RuntimeError('Size should be less than 1MiB')
     return b''.join([six.int2byte(random.randint(0, 255))
                      for i in range(size)])