Add whitelisting docker registries

[tpoitras]

Tagging @rhatdan for Tech Review:

Added details on whitelisting docker registries as per https://bugzilla.redhat.com/show_bug.cgi?id=1307210

Pretty rendered docs build:

http://file.bne.redhat.com/tpoitras/2016/Mar15/openshift-enterprise/whitelisting-BZ1307210/install_config/install/docker_registry.html#whitelisting-docker-registries

Questions:

1. Would this apply to OpenShift Origin as well?
2. Do we really need the = sign in between the --add-registry command and the specified registry? Example from my docs update, based on comment#1 in the BZ:

ADD_REGISTRY='--add-registry=registry.access.redhat.com

Because on my test machine, my /etc/sysconfig/docker file already has this line:

ADD_REGISTRY='--add-registry registry.access.redhat.com'

[runcom]

Would this apply to OpenShift Origin as well?

I guess this is up to the docker daemon running on the instances. I'll defer the answer to who know this better since I'm not familiar with Origin.

Do we really need the = sign in between the --add-registry command and the specified registry? Example from my docs update, based on comment#1 in the BZ:

No, we don't really need the equal sign at all, but docker options take it, just a matter of style, but both with and without sign are accepted options

cat /proc/`pidof docker`/cmdline 
/usr/bin/docker daemon --add-registry localhost:5000 --block-registry all

[runcom]

changes LGTM

[tpoitras]

Tagging team for peer review: @adellape @bfallonf @tnguyen-rh @ahardin-rh

Assuming this applies to Origin as well as Enterprise, maybe @adellape knows since he's a bit closer to Origin than me?

[bfallonf]

I think this would need ===='s around it? Example tags?

[tnguyen-rh]

@tpoitras One nit, otherwise LGTM.

[tpoitras]

Updated as per peer review. Thank you, @bfallonf & @tnguyen-rh

Good to merge once Travis is done the build confirmation.

[tpoitras]

[rev_history]
|link:../install_config/install/docker_registry.html[Installing -> Deploying a Docker Registry]
|Added details on whitelisting Docker registries.
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