Add nova-multi-cell job in experimental pipeline

Nova does not support few operation for exmaple
migration, unshelve etc cross cell and we need to
skip such operation testing for cross cell job.
nova-multi-cell job is multi cell job which will be
helpful to check the new tests in tempest.

Adding it in experimental queue so that we can run it
on demand basis.

Change-Id: Ib59f00f78921afd00a7dce26cfd453288eea462c
1 file changed