[blueprints] Support automatically creating a workspace and mounting it in instance

[townsend2010]

Blueprints need the ability automatically create a workspace and automatically mount that in the created instance.

The created workspace on the host should be in the form $HOME\multipass\$instance-name and should be mounted in the instance in the same manner.

The key in the yaml should look something like:

instances:
  foo:
    workspace: true