Should virtual host-specific definition import exclude virtual host name?

[michaelklishin]

See #10068 for the background.

During that investigation we have learnt that in early 2016 in 8ba0e07 a decision was to exclude the vhost field when exporting definitions of a single virtual host. I don't remember the details but the goal
could have been to allow for such files to be imported into any virtual host in a different cluster.

We can restore the vhost field to avoid the confusing scenario in #10068 and possibly ignore it during the import operation.