XD-444 Make External Server Rules Optional

When Redis and/or Rabbit is not available (e.g. on developer
machine), the tests should not fail.

However, if the servers are not available on the CI server, the
tests should fail.

Add an environment variable XD_EXTERNAL_SERVERS_REQUIRED. Gradle
passes environment variables to worker tasks.

When true, the tests will fail; when false, the tests will be
skipped.

Set the environment variable on the CI build plan to true.

Fix all Redis tests to use the RedisAvailableRule. Some tests
needed @Configuration of the connection factory because when
declared in XML it eagerly connects to Redis.