gremlin: add Sum() step for flows
gremlin: add Sum() step for flows

This step called on set of flows, aggregates integer metric described by
parameter, for example:

G.Flows().Sum('Metric.ABBytes') returns total ABBytes of G.Flows()

Change-Id: Ib89a648887d14301a41065ddfd7b832d701a9fff
Signed-off-by: Konstantin Dorfman <123db30ff0b691a58cf0ff9819721ec6c3581334@il.ibm.com>
Reviewed-on: https://softwarefactory-project.io/r/5530
Reviewed-by: Sylvain Baubeau <b0a7dc08a3f38d713e107103b0a85b0badc14b84@redhat.com>
Tested-by: Sylvain Baubeau <b0a7dc08a3f38d713e107103b0a85b0badc14b84@redhat.com>
Workflow: Sylvain Baubeau <b0a7dc08a3f38d713e107103b0a85b0badc14b84@redhat.com>

skydive-project/skydive,safchain/skydive,lebauce/skydive,lebauce/skydive,skydive-project/skydive,skydive-project/skydive,skydive-project/skydive,redhat-cip/skydive,lebauce/skydive,safchain/skydive,redhat-cip/skydive,safchain/skydive,skydive-project/skydive,safchain/skydive,safchain/skydive,redhat-cip/skydive,lebauce/skydive,skydive-project/skydive,lebauce/skydive,safchain/skydive,lebauce/skydive,lebauce/skydive,safchain/skydive,lebauce/skydive,safchain/skydive,skydive-project/skydive