user@juju:~$ juju bootstrap mainmaas --constraints tags=virtual
Creating Juju controller "mainmaas" on mainmaas
Looking for packaged Juju agent version 2.1.2 for amd64
Launching controller instance(s) on mainmaas...
ERROR failed to bootstrap model: cannot start bootstrap instance: cannot run instances: cannot run instance: No available machine matches constraints: [('agent_name', ['8fbd2913-785f-42d5-8b52-11514160e07f']), ('mem', ['3584']), ('tags', ['virtual']), ('zone', ['default'])] (resolved to "mem=3584.0 tags=virtual zone=default")