c8y devicemanagement certificates create: default to common name for the cert name

[reubenmiller]

It should be possible to upload a trusted certificate without specifying the --name. The default --name should be the Common Name (CN) of the given certificate.

c8y devicemanagement certificates create --file ./tmp/tedge-certificate.pem

[reubenmiller]

This is slightly more difficult to implement as first thought. It requires a new mechanism to allow setting two fields from a single input, whilst still allowing the user to override the name with an explicit value.