Update DIB doc
Update DIB doc

bin/disk-image-create was moved in a patch last year, this moves
disk-image-create to a regular python entry-point[1].Resulting in
our current document will be some problems. It seems that an
effective solution is to install DIB. Updated the relevant
information.

[1]:https://review.openstack.org/#/c/367157/

Change-Id: Idf1c15491ca77f4bd06a7b13aec676c84d15a800

zhangg/trove,zhangg/trove,openstack/trove,openstack/trove