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

doc/source/configuration.rst

@@ -83,7 +83,7 @@ Options
    they exceed this value.  This option specifies how many will be
    kept (usually you will see one more, as deletion happens before
    starting a new build).  By default, the last 7 old build logs are
-   kept.
+   kept.  Set this to ``-1`` to disable removal of logs.
 
 .. attr:: zookeeper-servers
    :type: list

nodepool/builder.py

@@ -592,6 +592,8 @@ class BuildWorker(BaseWorker):
         return log_dir
 
     def _pruneBuildLogs(self, name):
+        if self._config.build_log_retention < 0:
+            return
         log_dir = self._getBuildLogRoot(name)
         keep = max(self._config.build_log_retention, 1)
         existing = sorted(os.listdir(log_dir))

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.