Exclude SNAPS smoke for lxd scenario

Exclude SNAPS smoke for lxd scenario

SNAPS healthcheck is already excluded due to the
use of QCOW2 images, not compatible with lxd
scenarios

2 JIRAs have been cretaed to adapt the SNAPS cases
to lxd in Euphrates
https://jira.opnfv.org/browse/FUNCTEST-752
https://jira.opnfv.org/browse/FUNCTEST-753

Additionnal cleaning done on testcases.yaml
- remove juju_epc case (no one from OAI available
for Danube)
- remove orchestra (as standalone test case of the
MANO stack, postponed to Euphrates)
- comment aaa (empty sample used to illustrate
the VNF abstraction but not completed for Danube)

Change-Id: Ibad6cad4c92ee9897b6665b29c09b62cb1986aae
Signed-off-by: Morgan Richomme <morgan.richomme@orange.com>