Fix TTL on PKI certificate create to use string type and not int.

[jrasell]

The certificate creation call can uses a string such as 720h for
setting the TTL and shouldn't use an int.

Testing this locally the certificate creates successfully with the
following validity:

Not Before: Feb 21 15:25:01 2019 GMT
Not After : Mar 23 15:25:31 2019 GMT

Closes #313

[tyrannosaurus-becks]

Hey @jrasell! Thanks for submitting this! Would you be willing to add test coverage to those fields? I recently added info on how to do that by adding a CONTRIBUTING.md file.

[cvbarros]

Hello @jrasell , like this one, there has been several bugs regarding the handling of TTL inside the provider. Due to lack of availability, I haven't taken a stab across equalizing all TTL treaments, but that's something I've been planning to do.

If you see #255 changes where this was introduced, there is proper validation for a duration string. This treatment was derived from Vault API code itself, so it matches what upstream expects.

Also, I'd like to call out that a change in schema.TypeInt -> schema.TypeString . would be a breaking change, thus state migration is advised.

[tyrannosaurus-becks]

@jrasell fantastic! Thank you!