Tags are not implemented in the API itself, so they're not accessible from a nova CLI command.
Metadata is available to set key=value identifiers on your servers, using the nova CLI. These won't appear as tags in your control panel though. Here's some info about setting metadata on your servers with the nova CLI.
nova meta <server> <action> <key=value> [<key=value> ...]
Set or Delete metadata on a server.
Positional arguments:
<server> Name or ID of server
<action> Actions: 'set' or 'delete'
<key=value> Metadata to set or delete (only key is necessary on delete)