restructure and document e2e "focus" targets

[mythi]

our e2e tests have evolved a lot since they were originally created and we currently miss consistent structure how the tests are run with user defined "focus" (as in `-ginkgo.focus").

this task is to define the structure and get the restructuring done (including documentation)

corner cases that are currently not well defined:

• run selected devices with a subset of tests, for example:
  • without demo apps to just test plugin resources are OK
  • selected demo apps for a subset of devices in the same category (QAT Gen2 vs Gen4)
• run operator based tests only for certain devices

[mythi]

@ozhuraki @hj-johannes-lee FYI. This is related to your e2e activities

[mythi]

this task is to define the structure and get the restructuring done (including documentation)

looks like there's going to be a need to have NFD deployed as part of the suite. One idea would be to run and Skip() tests based on node features.