Fixed wording and reorganised Block Storage intro
Fixed wording and reorganised Block Storage intro

As per the bug, updated the wording and slightly reorganised
the Block Storage intro. Watch this space for the rest of the book.

Change-Id: I38d1a6c0c9dc093235db0573399fab459a5bc1db
backport: havana
Partial-Bug: #1251195

Akasurde/openstack-manuals,kairoaraujo/openstack-manuals,gongysh2004/openstack-manual,openstack/openstack-manuals,openstack/openstack-manuals,gongysh2004/openstack-manual,saeki-masaki/openstack-manuals,kairoaraujo/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,saeki-masaki/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,Akasurde/openstack-manuals,potsmaster/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,potsmaster/openstack-manuals