Enable users to supply their tailoring separately from the content.

[jan-cerny]

We require the tailoring file must be located under /tmp/openscap_data. However this is not documented and is not user friendly. We should allow arbitrary path I think. Another solution could be to document this requirement everywhere.

Relates to https://bugzilla.redhat.com/show_bug.cgi?id=1479008

[matejak]

Currently, we accept tailoring only if it is part of the content-tailoring archive. If the tailoring is already present in the system, or one would like to use existing content with a custom tailoring, we don't support these requests.