build-debian-package: Changed the way used to get the agent path

There is a configuration file in buildpackage that sets the
agent path. We source this file to get that path.

Acked-by: Danilo K. S. Yorinori <danilok@c3sl.ufpr.br>
Acked-by: Diego Giovane Pasqualin <dgp06@c3sl.ufpr.br>
Signed-off-by: Ricardo Tavares de Oliveira <rto07@c3sl.ufpr.br>