rhcos: Bump bootimage to 410.8.20190325.0

[cgwalters]

The "bootimage" is the one we use before pivoting. Bump this
so we don't need to support RHEL7 anymore even there.

This rolls in #1423

[cgwalters]

The MCD logs look good (though currently this is a small downgrade, we need to bump machine-os-content again)

I0325 23:31:39.841650    4868 run.go:22] Running captured: rpm-ostree status
I0325 23:31:39.879847    4868 daemon.go:738] State: idle
AutomaticUpdates: disabled
Deployments:
* pivot://registry.svc.ci.openshift.org/ci-op-phzqnhlf/stable@sha256:bdecb0f98ba80d609b9606c0d21af8b4fce27aec8313cb5ba088243f1bc48c3c
              CustomOrigin: Managed by pivot tool
                   Version: 410.8.20190322.0 (2019-03-22T20:35:08Z)

  pivot://docker-registry-default.cloud.registry.upshift.redhat.com/redhat-coreos/ootpa@sha256:84b54f4578bd04aa4e6fae2a7052a3b6df0bf5405dfbba1db44d59d282f3ee2f
              CustomOrigin: Provisioned from oscontainer
                   Version: 410.8.20190325.0 (2019-03-25T18:27:58Z)

[cgwalters]

Rebased to drop the dependency on #1423 for now per side discussion with wking.

[wking]

/lgtm

[ashcrow]

LGTM

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[ashcrow]

ART should have a build up for the machine-os-content soon-ish.

/cc @tbielawa @sosiouxme

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@cgwalters: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
ci/prow/e2e-aws	3026ec9	link	/test e2e-aws

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