Add vault file handing to secret function

[tomwerneruk]

Proposed fix for #1489 to load file contents when an attribute is prefixed by an @ sign.

[hashicorp-cla]

All committers have signed the CLA.

[eikenb]

Hey @tomwerneruk,

Sorry for the long sleep on this issue. I was finally getting around to reviewing and including the community PRs and this one looks good except for a missing test. If you are still interested would you consider adding a test? Thanks.

[angrycub]

I was looking at this and thought it looked interesting, but could you accomplish the same behavior with a template variable, file, and secret?

I'd be worried about keeping the @-reference safe, since some implementations that depend on CT explicitly deny file, but you wouldn't be able to flag out this behavior.