Add 'log4j2' domain tests to the Log4j 2.x agent

The Log4j 2.x agent now supports registering with 2 different tracing
domains, the existing 'log4j' domain and the new 'log4j2' domain.

The following tags were added to filter the tests when running the test
suite :

  - agent:jul
  - agent:log4j
  - agent:log4j2
  - domain:jul
  - domain:log4j
  - domain:log4j2
  - benchmark

For example, we can exclude all the log4j2 agent tests when testing a
version of LTTng that doesn't implement it :

  mvn clean verify -Dgroups='!agent:log4j2'

Or to run only the log4j domain tests on all agents that include them :

  mvn clean verify -Dgroups='domain:log4j'

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>