Add passing a logging level to processutils.execute

Previously, processutils.execute always logged what commands were
run at DEBUG level.  This is usually fine, but sometimes the caller
wants to log particularly important commands at INFO level instead,
and oslo should allow that.

This is oslo-incubator aa5b65882.  No other significant changes had
been made to processutils in oslo since the last sync.

Change-Id: I09dad3759bf2f759b297ee19205cf0460b9a6baa
Merge "Add passing a logging level to processutils.execute"