Fix bug 1665 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: I2d48ab18fa20708ff2697b2c0d9c2ef5c9fba7b9
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>