Docs: Add information on vagrant-ganeti test cluster

refs #15273
Docs: Update test_cluster usage
Docs: Emphasize node names
Docs (test_cluster): Add information on updating
Docs (test_cluster): Fix spelling, and grammar.
Merge branch 'enhancement/15273' into develop

* enhancement/15273:
  Docs (test_cluster): Fix spelling, and grammar.
  Docs (test_cluster): Add information on updating
  Docs: Emphasize node names
  Docs: Update test_cluster usage
  Docs: Add information on vagrant-ganeti test cluster