Driver interface's validate should return nothing

Some of the driver interface's validate(), eg pxe.VendorPassthru.validate()
returns True. The expected behaviour is for them to raise an exception if
validation isn't successful, or return nothing if validation succeeds.

This is fixed.

The docstrings for the validate functions in the base class have been updated
to mention that they should return None.

Change-Id: Ic8fb626874ed6cb11494c84dc184d70686441b0f
Closes-Bug: #1288363
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 "Driver interface's validate should return nothing"
Merge "Let ipmitool natively retry commands"