Fix bug 191050:Service principal accounts cannot use Key Vault via Po…

[YuhuiZhong]

…werShell

Update key vault data plane authentication logic to supports service principal

Test script cleanup

[azurecla]

Hi @YuhuiZhong, I'm your friendly neighborhood Azure Pull Request Bot (You can call me AZPRBOT). Thanks for your contribution!

It looks like you're working at Microsoft (yuhuiz). If you're full-time, we DON'T require a contribution license agreement.

If you are a vendor, DO please sign the electronic contribution license agreement. It will take 2 minutes and there's no faxing! https://cla.azure.com.

TTYL, AZPRBOT;