software/slaprunner: drop ssh key authority

These keys are not managed by trust of a certificate authority, just by
"trust of first use" so it does not make sense to use a key authority.

This also cause difficulties to publish the key fingerprint as a
parameter, because we can't get the key fingerprint until the authority
service is started.

Also enable ecdsa key.

This fixes random failures with slaprunner tests, because the published
fingerprint was never correct on first buildout run.

Existing webrunners will have a new ssh host key after this.