Fix getStringList in GerritConfig wrapper class

When getList from secure store class returns empty list try to get value
from gerrit.config. JGit getStringList will return empty list for not
found value therefore it is safe to ask base config file for this
property.

Change-Id: Iee8b6256a5ab7a1de698c46d2415a5a272f7e0c0
Signed-off-by: Dariusz Luksza <dariusz@luksza.org>