Replace deprecated imagePullSecrets helm setting

[Dean-Coakley]

Proposed changes

In order to be listed in the Rancher App Catalog we need to be able to set imagePullSecrets so that deployments created by Rancher can pull from private docker registries.

However, Rancher only support the following datatypes: string, multiline, boolean, int, enum, password, storageclass, hostname, pvc, secret.

This does not allow to specify a string array as required. This PR changes to accept one string instead of an array. This doesn't affect any known use cases of this field.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto master
• I will ensure my PR is targeting the master branch and pulling from my branch from my own fork

[pleshakov]

@Dean-Coakley
will this be added to the release 1.9 branch also?
If that is the case, it's better to have both the old and new parameters, so that we don't introduce any breaking changes in 1.9.* releases.

[Dean-Coakley]

@pleshakov Yeah that works for me.

Then I assume we are in agreement that the field controller.serviceAccount.imagePullSecrets will be removed in 1.10 release.