Make use of configurable global `Directory` variable

[cirocosta]

Hey,

right now we advertise the possibility of setting Directory as the default directory to be used as cwd but we're not actually using it at the directory resolution method.

Assuming that we have a cr file as follows:

---
Runtime:
  Directory: '/var/run/cr'
Jobs:
  - Id: 'Job1'
    Run: 'echo Job1_pwd=$(pwd)'

  - Id: 'Job2'
    Directory: '/tmp'
    Run: 'echo Job2_pwd=$(pwd)'

The result from the execution would be something like:

Job1_pwd=/var/run/cr
Job2_pwd=/tmp