Add apiserver.persistence config stanza to helm chart

[munnerz]

What this PR does / why we need it:

Allows persisting apiserver state with a pvc.

Which issue this PR fixes: fixes #135

Release note:

Add option to use PVC for persisting apiserver etcd data

[munnerz]

In future we should expand the helm chart to support specifying a custom etcd url to connect to, in order to support HA apiserver configurations (which should be the recommended deployment method)

[munnerz]

Re: whether we should not include any provisions for local etcd at all:

Okay sure, but then we need to remove all support for running etcd for Navigator and instead just expose a --etcd-servers, client-cert etc configuration option in the chart. (e2e tests will then also need appropriately updating to run etcd).

Given the early nature of Navigator, and the fact a user can run etcdctl backup in order to snapshot etcd (in case they want to switch to a clustered version), I figured this was a good stop gap. Currently the only way someone can persist data is to fork our Helm chart.

[jetstack-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kragniz

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[jetstack-ci-bot]

/test all [submit-queue is verifying that this PR is safe to merge]

[kragniz]

/test e2e v1.7

[jetstack-ci-bot]

/test all [submit-queue is verifying that this PR is safe to merge]

[jetstack-ci-bot]

Automatic merge from submit-queue.