Adds ability to override cluster_addr

[adamstrawson]

We have a use case where we're required to use $(POD_IP) over $(HOSTNAME) for VAULT_CLUSTER_ADDR environment variable.

This change adds the ability to override this variable.

The contribution guidelines don't define how chart versioning works with Pull Requests, but happy to amend the PR if you advise what you would like included.

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[adamstrawson]

Related issue: #397

[adamstrawson]

Any chance of some eyes on this? It'll shortly become a blocker for us.

[llivingstone]

Bumping this for some eyes!

[iridian-ks]

This is exactly what we need as well and is also blocking us at the moment!

[adamstrawson]

Closing as a duplicate PR was created and merged #709