A server without TLS was added to serve the SCEP endpoints. According
to the RFC, SCEP has to be served via HTTP. The `sscep` client, for
example, will stop any URL that does not start with `http://` from
being used, so serving SCEP seems to be the right way to do it.
This commit adds a second server for which no TLS configuration is
configured. A distinct field in the configuration, `insecureAddress`
was added to specify the address for the insecure server.
The SCEP endpoints will also still be served via HTTPS. Some clients
may be able to work with that.
This commit also improves how the crypto.Decrypter interface is
handled for the different types of KMSes supported by step. The
apiv1.Decrypter interface was added. Currently only SoftKMS
implements this interface, providing a crypto.Decrypter required
for SCEP operations.
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