Allow 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.

Change-Id: I89af6fdc0bb11a4134aaaca417a54ba3b515ca7e
Merge "Allow passing a logging level to processutils.execute"