Fix a few issues in unittest data generation

This patch fixes two ugly but non-critical issues:

- the getFQDN wrongly generated only the first component as
  limited-length name, the rest could be any length, and that makes
  the generated name bad for display purposes
- the random generator had real issues in finding an offline instance
  with the right properties when starting from certain seeds (found
  accidentally); this meant the test took 1 minute, instead of ~300ms,
  for the node_addSecOffline

Furthermore, for the same evil seed, the Container_findByName test
wasn't able to generate enough good data, so we change its
implementation to directly generate an uniques list, instead of
generating random lists and filtering the unique ones.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>