Workaround for xenv issue with unicode in Python2

the reason for the problem reported in https://mattermost.web.cern.ch/lhcb/pl/cxargk9bopgiubscubzo3xswze is that the relocation value for a data package is set as unicode instead of str (Python2) and when trying to get the value we have a exception because the conversion to str of the internal variable object of xenv returns [u'a', u'_', u'v', u'a', u'l', u'u', u'e'] instead of 'a_value'.

This change force the value to str fixing the problem.