DRY setup for stemcell serverspecs

Signed-off-by: Jeffrey Peckham <peckham@pivotallabs.com>
Remove bosh_tripwire stage

* It wasn't being run since TW_LOCAL_PASSPHRASE was never set
* Removing tripwire package from new CentOS stemcell
* Leaving tripwire in Ubuntu stemcell, just in case...

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>