mockbuild: use mock from repo, improve configuring

Signed-off-by: Igor Gnatenko <ignatenko@mirantis.com>
scripts: pick DNS from config file

Signed-off-by: Igor Gnatenko <ignatenko@mirantis.com>
mockbuild: add POSTFIX

Signed-off-by: Igor Gnatenko <ignatenko@mirantis.com>
build-rpm: simplify by using ls instead of find

Signed-off-by: Igor Gnatenko <ignatenko@mirantis.com>