Make swift-form-signature output a sample form

swift-form-signature would give you the required expiration-time and
HMAC signature, but it wouldn't help you actually construct the HTML
form. To do that, you had to go look at the formpost middleware's doc
string and make up a form yourself.

For convenience, this commit makes swift-form-signature output a
sample form with the computed values filled in already; the user only
needs to fill in the Swift cluster's hostname.

Change-Id: I70d70a648b78b382dbfbe8ff918e6158a7f6a0ab
Merge "Make swift-form-signature output a sample form"