rbd: add support for rbd image resize
rbd: add support for rbd image resize

See gwcli and ceph-iscsi-ansible docs for how to resize
the lun and image with their tools.

For rbd/targetcli based updates do:

rbd resize --size=10G gwcli0

/backstores/user:rbd/gwcli0> set attribute dev_size=10737418240
Parameter dev_size is now '10737418240'.

Signed-off-by: Mike Christie <39daeb1549a929782e38aa87c4e3da7f3844b09a@redhat.com>

open-iscsi/tcmu-runner,agrover/tcmu-runner,open-iscsi/tcmu-runner