An update for devstack integration scripts

* Config file for 'devbox' installation mode added
* 'if' clauses added to detect if a component should be installed
* README updated
* A few Horizon config items moved to 'local_settings.py' file
* Use 'trueorfalse' when comparing 'string booleans'
* Prerequisites added
* debug/verbose settings added
* local.conf renamed to prevent accidential overwriting existing config

Change-Id: I126196b64343f133e0f5e311dfa74101a1374c89