don't check minimum key size when disabled (#1754)

don't check minimum key size when disabled (#1754)

* cleanup old comments for ed25519

These comments were added when x/crypto/ed25519 could not yet handle
ed25519. It does now, so it should be removed.
Also the key type is now replaced with the proper constant.

* move the minimum key size config before the check

This moves the actual config lookup before any check is done. This
avoids problems with calling to ssh-keygen which doesn't support the
expected output format and returning an error, when the check is disabled.