[Storage-backend][vault pki] Add storage backend vault pki to template certs from vault

[tw3rp]

This PR includes changes to add a new storage backend vault pki to template certs from vault

1. "Inherits" all the methods from the vault backend so authentication and all the flags remain the same
2. Certs are issues by listing keys as /mountpath/rolename/commonname eg. /pki/issue/my-role/www.example.com and the keys returned are /pki/issue/my-role/www.example.com/certificate and /pki/issue/my-role/www.example.com/private_key. This is so because it goes against confd design principles to pass in multiple arguments to a key
3. The cert and the key must be templated to the same file because it is an open issue to template to multiple files (Create multiple config files from templates #256)
4. An example script is put in so we can split them into individual files once templated.

This is for issue #731 to template certs as a new backend

[tw3rp]

small write up on how it works https://rprabhu.dev/2019/vault-confd-automation/