Let ipmitool natively retry commands

Instead of calling ipmitool multiple times on failure via
  utils.execute(*args, attempts=3)
allow ipmitool to use its own native retry behavior with -N.. -R..
if those options are supported by the installed version of ipmitool.
This will fall back to a single run of ipmitool on older versions,
which should be fine -- it defaults to retry several times anyway.

This patch adds a configurable min time between retries, which is used,
in conjunction with the ipmi retry time, to determine these option's
values. It will be further leveraged in a subsequent patch as well.

It also adds a note in the deployer docs about known issues with
the openipmi project.

Change-Id: I7a4ff941144a03bd441459561efb68760391da1a
Partial-bug: #1320513
Merge "Let ipmitool natively retry commands"