Run tmt tests using local file in the Api machine

[pellecchialuigi]

Current tmt-plan is only able to run tmt tests from remote repository.
It will be useful to let users able to run tests from local files, when files are in the machine running the Api.

[pellecchialuigi]

#77 solve that issue

Added plan_type == local in the tmt file to enable a different discover section that is using local files.

To be able to run a test case from the machine that is running the API project (e.g. one provided in the example folder) you can set:

plugin: tmt
context: plan_type=local
repository: /BASIL-API
relative path: /examples/tmt/local/tmt-dummy-test