Added possibility to use non-existent keys

When including a file, it failed if any of the keys inside it did not exist, now
it will replace the non-existing keys with '' instead, allowing the usage os
non-existing keys.

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

Now you can organize your yaml files into subdirectories and apply them
all just passing --recursive option or adding the proper configuration
file option

Change-Id: I126751e347622716c592c6ed1a57b8acb7bf79a4
Signed-off-by: David Caro <dcaroest@redhat.com>