Mark 'manager' as primary machine

Mark 'manager' as primary machine to be able to use commands like
'vagrant ssh' without specifiying the machine name.

This will solve the issue 'This command requires a specific VM name
to target in a multi-VM environment.'.

Change-Id: I15143d54d69dc05146aa141594924e30d18ba54e
Merge "Mark 'manager' as primary machine"