reservations: get machine-readable place name

[Bastian-Krause]

When using labgrid's reservation feature..

$ eval `labgrid-client reserve --shell board=imx6`
$ labgrid-client wait $LG_TOKEN
owner: erbse/bst
token: L7KN0X2UUG
state: allocated
filters:
  main: board=imx6
allocations:
  main: example-board
created: 2023-07-21 12:44:02.284611
timeout: 2023-07-21 12:45:04.489250
$ export LG_PLACE=+
$ labgrid-client lock
acquired place example-board

..there's no way of reading the actual place name (example-board in the example above) in a machine-readable way.

Maybe something like labgrid-client wait --shell (similar to labgrid-client reserve --shell) could be implemented.