increase disk_erasure_coconcurrency

When we added concurrent disk erasures, we kept the concurrency
to 1 as to not risk any different oeprator behavior, at the cost
of not faster erasure times.

That being said, we have had the setting in place for some time
and we have received no reports of issues, so we are incrementing
it to four as that should be still quite relatively safe from a
concurrency standpoint for disk controllers in systems.

Change-Id: I6326422d60ec024a739ca596f46552bbd91b0419

ironic/conf/deploy.py

@@ -108,7 +108,7 @@ opts = [
                        'state. If True, shred will be invoked and cleaning '
                        'will continue.')),
     cfg.IntOpt('disk_erasure_concurrency',
-               default=1,
+               default=4,
                min=1,
                mutable=True,
                help=_('Defines the target pool size used by Ironic Python '

releasenotes/notes/maximum-disk-erasure-concurrency-6d132bd84e3df4cf.yaml

@@ -0,0 +1,10 @@
+---
+other:
+  - |
+    The maximum disk erasure concurrency setting,
+    ``[deploy]disk_erasure_concurrency`` has been incremed to 4.
+    Previously, this was kept at 1 in order to maintain continuity of
+    experience, but operators have not reported any issues with an increased
+    concurrency, and as such we feel comfortable upstream enabling concurrent
+    disk erasure/cleaning. This setting applies to the ``erase_devices`` clean
+    step.