Libraries improvements, Guest cleanup, variables from files...

• Move libraries handling into a separate module
• Adjust loading variables from YAML files [#316]
• Support environment variables from YAML files
• Give a nice error for expired kerberos [fix #57]
• Merge Guest relocation and documentation [#307]
• Describe essential Guest methods in more detail
• Update test import story and documentation
• Merge extra-task as summary in test export [#304]
• Move default plan handling into a single method
• Move the Guest class from base to steps.provision
• Save root in run.yaml
• Document L1 metadata defined in the discover step
• Improve Makefile editing during test import
• Use extra-task as summary in test export
• Mention default methods in the step help message
• Handle invalid url when library provided as fmf id
• Allow library git clone to fail