[FEATURE] Allow use of existing private registry secret

[pulberg]

Is your feature request related to a problem? Please describe.
When using a private registry, the installation requires the private registry user/pass to be entered and the name of the secret that will be created. I already have a cluster-wide (all namespaces) private registry secret, it doesn't make sense to have to create a new one for the Longhorn installation.

Describe the solution you'd like
The installation will let me enter the name of the existing private registry secret to use.

[PhanLe1010]

Sure! That makes sense.

[kaxing]

This was validated within v1.2.0 release test cycle: #2432 (comment)

But the current function says it will auto-generate new secret rather giving an option as requested in the original description. cc @innobead

[innobead]

This was validated within v1.2.0 release test cycle: #2432 (comment)

But the current function says it will auto-generate new secret rather giving an option as requested in the original description. cc @innobead

@kaxing u should verify on 1.3.0 instead of 1.2.x. I would suggest to wait to test until we have rc1 chart.

[yangchiu]

verified passed on master-head by importing repo: longhorn/longhorn/chart master-head into rancher (using https://github.com/yangchiu/rancher-charts/tree/longhorn-v1.3.0) and try to install the imported latest longhorn.

#3954 is needed since there's some incorrect indent and missing double-quote on the original master-head cause the questions unable to be displayed while the installation.

with #3954, the "using existing private registry secret" options displays and functions as expected.