Make swift-form-signature testable

Moved the body of bin/swift-form-signature into
swift/cli/form_signature.py, like was done with swift-ring-builder and
others. Added a couple basic tests; there's not 100% coverage, but
it's better than the 0% coverage we had before.

It's almost a straight forklift, but I changed exit() calls to return
statements.

Change-Id: Ie2f702c070da24d9cdface83b9e838e9e2965085
Merge "Make swift-form-signature testable"