Enhancement: Allow specifying user and host when creating keys

When adding a new key to the repository, the username and hostname for the new
key can be specified on the command line. This allows overriding the defaults,
for example if you would prefer to use the FQDN to identify the host or if you
want to add keys for several different hosts without having to run the key add
command on those hosts.

https://github.com/restic/restic/issues/2175
