Variable for SCAP files in openscap plugin

[hbraswelrh]

Is your feature request related to a problem? Please describe.

With the removal of the manifest file, we missed some plugin configuration options, including the option to define where to locate the SCAP files used for tailoring and scan.

Describe the solution you'd like

In the new approach we have the concept of variables tied to targets. We can create an optional variable used by openscap plugin.

Some examples of plugin specific  variables can be found in ampel-plugin.

[hbraswelrh]

@marcusburghardt @jpower432 From community meeting - review configuration & variable information.

[marcusburghardt]

@marcusburghardt @jpower432 From community meeting - review configuration & variable information.

During the meeting @jpower432 found that the variable is already available to be used, which is great.

I believe this issue can be considered solved now. Maybe we could only assess if the documentation needs any update.
For example, mentioning some use cases we discussed in the meeting, such as using a customized Datastream or a Datastream provided by any other form different than the official scap-security-guide RPM.