Improve backup LU testing
Improve backup LU testing

With all the changes to the backup LU, some proper testing is in order
to ensure we can try and export disks in all situations. This patch
covers all the new use cases.

Signed-off-by: Hrvoje Ribicic <be5828fca31f89218048bec4189e30a581aa65da@google.com>
Reviewed-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>

yiannist/ganeti,ganeti/ganeti,ganeti/ganeti,andir/ganeti,bitemyapp/ganeti,leshchevds/ganeti,mbakke/ganeti,yiannist/ganeti,ganeti/ganeti,andir/ganeti,leshchevds/ganeti,mbakke/ganeti,dimara/ganeti,onponomarev/ganeti,grnet/snf-ganeti,andir/ganeti,dimara/ganeti,yiannist/ganeti,onponomarev/ganeti,grnet/snf-ganeti,bitemyapp/ganeti,leshchevds/ganeti,mbakke/ganeti