Make it easier to mount known_hosts from a ConfigMap in Helm chart

[squaremo]

Addresses #1113, and helps with #1101 by making the chart usable from the integration test (I think).

WIP until I've actually tried the Helm chart install myself.

[squaremo]

Note to self, this may need to alter the howto bit of setup.md, and possibly FAQ.

[stefanprodan]

LGTM

[stefanprodan]

@squaremo you could also bump Flux version to 1.4.0

[squaremo]

you could also bump Flux version to 1.4.0

After trying this with Helm, I've realised it needs a release after 1.4.0, since it will mount a ConfigMap over the top of ~/.ssh, and that will break any flux image that doesn't include the change from the first commit. -> I'll do a PR that just includes that first commit (which is safe and won't break existing configs), then we can update the examples and Helm chart once there's a new release out.

[squaremo]

Note to self: PR for the first bit is #1154. Once that's merged and in a release, rebase this and bump version in Helm chart.

[squaremo]

I removed the change to the example -- it can stay commented out. So this is just about making it possible to tell the Helm chart to include a known_hosts file. @stefanprodan Do you mind checking it again / trying it out? Thank you!