Customize the volume list per layer

- Create an input yaml file for the list of volumes that will be mounted
to the container when running the tests.
- Customize the list of containers for each layer.
- Add the posibility to store the bluval-<blueprint>.yaml file outside
the validation project and mount it at runtime.

Signed-off-by: Cristina Pauna <cristina.pauna@enea.com>
Change-Id: Id3c21cb861879cd3040b06badfb9622429734ce6