[stable/unifi] adding functionality to mount extra volumes

[miwinski]

Is this a new chart

existing chart [stable/unifi]

What this PR does / why we need it:

This PR allows to specify additional volumes to be mounted to unifi container during deployment of Unifi Controller. So for example - my environment is using a single
Unifi switch between my k8s cluster and my NAS (volumes are exposed via iSCSI). If I use persistence.existingClaim=iscsi-pvc-for-unifi and during normal operations of Unifi I do
a firmware upgrade of that switch I'm disrupting connectivity between Unifi Controller
and remote storage potentially causing corruption of controller's database stored on remote
volume. With this PR I can specify persistence.existingClaim=unifi-pvc-on-local-node-disk which will allow me to rebuild/upgrade Controller POD without loosing data and mount additional remote volume for example a backup dir. This way I only store backups on remote drive and a short network disruption doesn't have that big impact on Unifi controller.
Another benefit of this PR is that it also allows to specify another extra volume and mount a configmap serving a custom config.gatewy.json.

Which issue this PR fixes

No issue reported for this feature

Special notes for your reviewer:

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• DCO signed
• Chart Version bumped
• Variables are documented in the README.md
• Title of the PR starts with chart name (e.g. [stable/mychartname])

[k8s-ci-robot]

Hi @iwwwwwwi. Thanks for your PR.

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[miwinski]

/assign @billimek

[billimek]

/ok-to-test

[billimek]

Hi @iwwwwwwi. Thanks for the PR!

Do you mind bumping the chart version to 0.9.0 as there is a new feature being added?

[miwinski]

@billimek bumped version to 0.9.0

[billimek]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: billimek, iwwwwwwi

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