Fix datasetService to find tempest log paths
Fix datasetService to find tempest log paths

Prior to this patch, datasetService did not find the log paths that
'stackviz-export' and 'gulp data' generated. This commit adds the
prefix 'data/' to all GET calls in datasetService to ensure that the
log files are found.

Change-Id: Ie88d30a0844cea57cefed6b22265ccfd6050b3a9

openstack/stackviz,openstack/stackviz,openstack/stackviz