python3 bytes fix that actually works

Prior to this get_cert was broken.
Include the ssh cert serial in the URL so that users can have multiple
certs available in S3 at the same time (still will only have one
active/loaded cert in ssh-agent)
Include the ssh cert serial in the URL so that users can have multiple
certs available in S3 at the same time (still will only have one
active/loaded cert in ssh-agent)
Merge branch 'multiple-user-certs-in-s3' of github.com:synfinatic/ssh-ca into multiple-user-certs-in-s3
fix pep8
switch to using the envirtonment instead of serial
fix last pep8 error
need to be able to replace user public key certs
Merge pull request #8 from synfinatic/multiple-user-certs-in-s3

Multiple user certs in s3