Allow PKI certificate fields to be specified as an array

[sidewinder12s]

Feature Request:
Allow certificate fields to be specified as an array.

Currently if you try to use a comma in a certificates Organization or OU field it will split the content on the comma with no way to escape the comma.

Environment:

• Vault Version: 0.9.0
• Operating System/Architecture: Ubuntu 16.04

References:
Originally brought up here: https://groups.google.com/d/msg/vault-tool/P0DVffjaglY/Cp_fG6fjCQAJ

[jefferai]

Putting into 0.9.1 milestone but as previously communicated will probably slip.

[jefferai]

We should do this with OU as well.

[jefferai]

Note to Vault team: we have to be careful on upgrade here; we can't do it transparently on read since certs are issued cluster-locally.