(PCP-202) Module's interface: success when results are valid
(PCP-202) Module's interface: success when results are valid

Making it clear in the modules interface specs that pxp-agent will not
mark an action as failed by considering the action's exit code.

Instead, an action will succeed whenever its results on stdout are
validated by using the "results" JSON schema provided by the action's
metadata.

[skip ci]

puppetlabs/pxp-agent,puppetlabs/pxp-agent,puppetlabs/pxp-agent