BUG 484 - XmlReader improve readability

- XmlReader - improve readibility of methods isSimple(Composite) node by
  extracting common logic to method skipCommentsAndWhitespace.

The change comes from patch set
https://git.opendaylight.org/gerrit/#/c/7337/9
which was originally commited by Tom Pantelis. In this commit is selected
only one change which doesn't relate with BUG 484

Change-Id: Icef852da20e8f624b4bc3d61e334d88f4766474d
Signed-off-by: Jozef Gloncak <jgloncak@cisco.com>
Merge "BUG 484 - XmlReader improve readability"