fix next-pool overrides by job

[florian-at-bareos]

The next pool directive can be defined in the Pool and Job resource, the next pool directive in the Job resource should override the directive in the Pool resource. Currently, The directive in the job resource is not taken into account when running a job via the "run" command in the bconsole. This PR fixes this and takes the directive from the Job resource if specified.

Also, this PR adds a section in the copy-migrate systemtest to test the override behaviour of this directive.

Fixes #2253: Copy job fails, because "Next Pool" is not accepted as an override in the job config.

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[florian-at-bareos]

The code is still awkward because the ScanCommandLineArguments method in ua_run.cc also sets values that are not given in the command line, applying overrides.

[florian-at-bareos]

Also, is the name of the method ResetRestoreContext in ua_run.cc correct? The method updates jcr->dir_impl->res values from a RunContext reference, no RestoreContext at all.

[sebsura]

I have not tested this manually yet. Could you add a test that tests the overrides ?
We probably already have a suite that uses copy jobs, so feel free to add one testrunner each for the two cases.

[sebsura]

Looks good to me. Thanks for the work!