NEXUS-6123: Special validation for site.xml added
NEXUS-6213: Added log line when lax XML validation is used

As basically this means XML was detected as invalid, but
lax XML validation overrides that to "neutral".
NEXUS-6213: Laxed site xml path matching.
NEXUS-6213: UT

With positive and negative tests.
Merge pull request #313 from sonatype/NEXUS-6213-site-xml

NEXUS-6213: Validation for Maven Site XML