Demo app moves to different namespace

[bboreham]

Change kubeadm instructions to show use of sock-shop namespace rather than default.

Background: the demo installs a DefaultDeny network policy annotation, which is tripping up some users who go on to create some more pods without removing the annotation. This change matches microservices-demo/microservices-demo#339 which moves the whole demo out of the default namespace.

This change should not be merged until the microservices-demo change is merged.

CC @errordeveloper

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This change is

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CLAs look good, thanks!

[bboreham]

I signed a CNCF CLA as bryan@weave.works

The upstream change to the demo app is now merged so would be good to get this through.

[lukemarsden]

LGTM

@luxas PTAL

[luxas]

LGTM