Bug 576 Fixes of critical issues for YT components

Bug 576 Fixes of critical issues for YT components

This is fix for https://bugs.opendaylight.org/show_bug.cgi?id=576
Added Nonnull annotations and preconditions checks for annonymous apply methods.
Fixed never reachable if statement in writeValue(final XMLStreamWriter writer, final @Nonnull Node<?> data, final SchemaNode schema) in XmlStreamUtils
since instanceof is doing already null check.
Removed empty if statement in NormalizedNodeContainerModificationStrategy in verifyStructure method.
Removed dead code from in PerRepoTestRunner runnersFromFeatures method.

Change-Id: Icec2a30149a17d8b76497d95ac0054297369e985
Signed-off-by: Lukas Sedlak <lsedlak@cisco.com>