gluster: Support remote gluster server.

Sometime we have to use gluster in remote host.

Signed-off-by: Feng Yang <fyang@redhat.com>
virttest.qemu_storage: Support gluster in image_check().

Signed-off-by: Feng Yang <fyang@redhat.com>
virttest.qemu_storage: Do not check parent directory exist when glusterFS used

Got following error message in unattended_install case:
03/28 10:55:54 ERROR|qemu_stora:0128| Parent directory of the image file gluster://10.66.8.107:0/vol_test/rhel64-64.qcow2 does not exist
03/28 10:55:54 ERROR|qemu_stora:0129| This usually means a serious setup error.
03/28 10:55:54 ERROR|qemu_stora:0130| Please verify if your data dir contains the expected directory structure
03/28 10:55:54 ERROR|qemu_stora:0133| Backing data dir: /var/lib/virt_test
03/28 10:55:54 ERROR|qemu_stora:0134| Directory structure:
03/28 10:55:54 ERROR|qemu_stora:0136| /var/lib/virt_test
03/28 10:55:54 WARNI|qemu_stora:0138| We'll try to proceed by creating the dir. Other errors may ensue

This is some interference information.
I think we should not check parent directory when glusterFS used.
But we can check parent directory available when image_backend = filesystem.

Signed-off-by: Feng Yang <fyang@redhat.com>
Merge pull request #1556 from FengYang/gluster

gluster: Support remote gluster server.