Enable selection of first remote config mgmt admin subject-name from `step ca init`

[dopey]

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Issue details

https://github.com/smallstep/certificates/issues/962

Why is this needed?

Some CAs (e.g., those with name constraints) will not be able to generate a cert with the default admin name as a subject. Once created, these CAs will be impossible to configure remotely, without taking some manual steps.