Added possibility to use non-existent keys

Added a new configuration option under the section "job_builder" named
"allow_empty_variables" that if set to true, will replace non existing variables
in strings with the empty string instead of raising an error.

Change-Id: Iad9f0e522725e6fd6681cd62d3e36f69baf09585
Signed-off-by: David Caro <dcaroest@redhat.com>