Create an initial release payload as release:initial

[smarterclayton]

When we tag images in, create a second image stream stable-initial
that will not be modified and then let a secondary task create an
image stream from that alternate source. If RELEASE_IMAGE_INITIAL
is already specified, skip the action and instead tag it to that
location.

Simplify the structure of the AssembleReleaseStep to handle
both initial and latest.

Makes it easy to test an upgrade from the previous state to the
current state (post change) while also allowing for injecting an
initial step.

Fixed from revert in #290, was caused by this line:

	return fmt.Sprintf("[release:%s]", strings.ToUpper(s.tag()))

should be

	return fmt.Sprintf("[release:%s]", s.tag())

and caused [release:LATEST] to not be available to target from
--target [release:latest] like the job does.

[stevekuznetsov]

/lgtm
/approve

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This pull-request has been approved by: smarterclayton, stevekuznetsov

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