BUG-1382: eliminate use of QName.getPrefix from yang parser

Updated tests - removed tests on QName's prefix.

Please do not merge before https://git.opendaylight.org/gerrit/#/c/9640/ is merged.

Change-Id: I41b8940965631609074c5bb8e4c2e0538a33f6a9
Signed-off-by: Martin Vitez <mvitez@cisco.com>