doc: Common graph used in 2 quick start files

The graph in quick-ceph-deploy.rst applies to
quick-start-preflight.rst.
The graph in deploy seems more complete, so I put the common
documentation in quick-common.rst and had it included.
doc/conf.py has 'start/quick-common.rst' in exclude patterns so that
sphinx does not complain about this file not being in toc.

Signed-off-by: Kevin Dalley <kevin@kelphead.org>
Merge pull request #1786 from nereocystis/quick-common

doc: Common graph used in 2 quick start files