Add list of available places

[istepic]

Description

Clients sometimes want to know which places are available before acquiring the place.

Checklist

• Original documentation for the feature already covers this PR, documented on command line, as --acquired.
• Tests for the feature, no, it's a simple change
• PR has been tested

Testing:

$ labgrid-client places
target-d-8F94FBAC4EC2DAB50539F2499000055A
target-d-8F94FBAC4EC2DAB50539F299900003D5
$ labgrid-client places --acquired
target-d-8F94FBAC4EC2DAB50539F299900003D5
$ labgrid-client places --available
target-d-8F94FBAC4EC2DAB50539F2499000055A
$ labgrid-client places --help
usage: labgrid-client places [-h] [-a] [-l] [--sort-last-changed]

options:
  -h, --help           show this help message and exit
  -a, --acquired
  -l, --available
  --sort-last-changed  sort by last changed date (oldest first)