[BugFix] [RHEL/6] Various NIST SCAP content validation test suite fixes

[iankko]

Fixes:

• [RHEL/6] "xccdf_org.ssgproject.content_benchmark_RHEL-6 - The <xccdf:metadata> element SHALL be provided in the <xccdf:Benchmark> element. " #1041
• [RHEL/6] scap_org.open-scap_datastream_from_xccdf_ssg-rhel6-xccdf-1.2.xml - Every <xccdf:platform> or <cpe2:fact-ref> MUST match as EQUAL or SUPERSET to a CPE in a CPE dictionary component of this data stream.  #1042

[iankko]

For the failing Jenkins CI el6 job -- it's expected that it's failing:
    [1] https://jenkins.open-scap.org/job/scap-security-guide-pull-requests/397/consoleFull

This will fail till fix from #1048 isn't merged (since right now / with current master the make validate target for RHEL/7 is failing when built on RHEL/6 [with openscap-1.0.x]).

Once fix for #1048 is merged, the testing job for this PR should be rescheduled and it should pass in that moment (since the underlying changes are not related with OVAL checks).

[iankko]

re[test this please]

[iankko]

@jan-cerny
Honzo, could you possibly review this one too? (should be easy to review -- two small changes, pretty straightforward).

Thanks, Jan!

[jan-cerny]

@iankko Thank you for fixing those issues. LGTM. ACK.

[iankko]

Thanks too!