Allow disabling build-log-retention

This allows setting build-log-retention to -1 to disable automatic
collection of logs.  This would facilitate managing these logs with an
external tool like logrotate.  Another case is where you have the
builds failing very quickly -- say, one of the builds has destroyed
the container and so builds fail to even exec dib correctly.  In this
case it's difficult to get to the root-cause of the problem because
the first build's logs (the one that destroyed the container) have
been repead just seconds after the failure.

Change-Id: I259c78e6a0e30b4c0a8d2f4c12a6941a2d227c38

releasenotes/notes/retention-disable-ac0c43d3912c1311.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  - |
+    Setting ``build-log-retention`` to ``-1`` will disable the removal of
+    old build logs.