tough-kms: some repos cannot be loaded after signing with kms

[webern]

Occasionally, the KMS sign function produces a signature with a length shorter than the RSA key modulus used in signing. For example with an RSA_3072 key, most KMS signatures will be 384 bytes long. But approximately 1 out of 200 of signatures will be shorter than that.

Ring has a check to ensure that the length of the signature being verified exactly matches the length of the key modulus. This means that 1/200 KMS signings produces a signature that ring verify will reject.

Both KMS and openssl pkeyutl assert that these 'short' signatures are valid. To ensure that all of ring, KMS and openssl agree, we should prepend signatures returned from KMS that are shorter than the modulus length with zeros until the modulus length is achieved.

[webern]

This has been fixed in #263