After upgrading to 20.07.01, storagePrefix no longer validates

[stenwt]

Describe the bug
I upgraded from 20.01 to 20.07.01 because I was seeing #430 on my cluster (this arose suddenly, after 200+ days uptime). Now that I've upgraded to 20.07.01, my backend shows "status: failed". I attempted to re-create the backend, and I get:

...storage prefix may only contain letters/digits/underscore/dash

My storage prefix contains a . and has since I created it under 20.01.

Environment
Provide accurate information about the environment to help us reproduce the issue.

• Trident version: 20.07.01
• Trident installation flags used: -n trident
• Container runtime: docker 18.09.8
• Kubernetes version: 1.18.8
• Kubernetes orchestrator: Rancher 2.4.8
• Kubernetes enabled feature gates:
• OS: Ubuntu 18.04
• NetApp backend types: ontap-nas
• Other:

To Reproduce
Steps to reproduce the behavior:

Expected behavior
A clear and concise description of what you expected to happen.

Additional context
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[gnarl]

This fix is included in Trident v20.10.0.