evaluate values in metadata.yaml

[sopos]

I've noticed that some values might be process a bit more by tmt in the metadata.yaml file.

E.g. duration: 60m 5m

It is kept as a string while it would be beneficial to have the resulting value converted to seconds or so.

[pellecchialuigi]

I created a PR to add tests to duration adjust that are based on the value proposed by tmt test show. Today this command is providing a resulting string. If we plan to propose a value in second we should adjust those tests as well.