Use ar and tar to extract powerdns for CentOS

* `dpkg` is Ubuntu specific
* [Delivers #57416240]
`ar` and `tar` is a much fairer assumption about the stemcell instead of
`dpkg`, which is more Debian specific

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>
Remove unreachable code, set -e prevents this

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>
build_stemcell no longer idempotent, it is only called once

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>
Split Candidate & Local builds to build releases locally

* For running stemcell specs against local modifications
* [#57416240]

Signed-off-by: Tony Hansmann <thansmann@pivotallabs.com>