Option validator should use option sources
In https://github.com/theforeman/hammer-cli/commit/5bf50733c576963a316b6978954a1b775b810a1c Hammer introduced option sources that enable reading option values from additional providers. Default options have been turned into an option source too. Validator currently reads defaults directly. It should use option sources instead, otherwise it will prevent a command to start when required options come from some additional option source.