Changes

testenv: improve --config argument parsing

* Support using wildcards for the config names via fnmatch as that makes
  it much easier to run the ggsn tests against all osmo-ggsn config
  variations, and update the examples in "testenv.py -h" to illustrate
  this.
* Fix that it didn't complain about an invalid --config argument, as
  long as there was a valid --config argument before it.
* Let raise_error_config_arg only output the invalid --config argument
  instead of all of them.
* Complain if "--config all" is used in combination with another
  --config argument.
* Sort testenv*.cfg files found alphabetically, so they are always
  executed in the same order.

Change-Id: I66b976b0332be523c084a6b5d38d0f62134b495d