Before this commit, EAB keys could be used CA-wide, meaning that
an EAB credential could be used at any ACME provisioner. This
commit changes that behavior, so that EAB credentials are now
intended to be used with a specific ACME provisioner. I think
that makes sense, because from the perspective of an ACME client
the provisioner is like a distinct CA.
Besides that this commit also includes the first tests for EAB.
The logic for creating the EAB JWS as a client has been taken
from github.com/mholt/acmez. This logic may be moved or otherwise
sourced (i.e. from a vendor) as soon as the step client also
(needs to) support(s) EAB with ACME.
