test: ceph_test_rados_io_sequence - seq 14 - write object filling gaps

test: ceph_test_rados_io_sequence - seq 14 - write object filling gaps

New sequence - write object initially with gaps and fill in the holes
until the object is fully populated. Permutate the step (gap between
writes), start at randomized offsets to produce gaps at the start of
the object as well as between writes

Signed-off-by: Bill Scales <bill_scales@uk.ibm.com>