Fix bug 1664 to remove LW-UI from newtag.sh

The commit removes LW-UI from the set of repos where the tag
will be created.

It also checks whether the branch exists or not, skipping the tagging
if the branch does not exist in the repo.

Change-Id: I9ae444ffd27b3c970a472d741a90f56041cdb802
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>