Improve NetworkServiceTestCase._get_ip_flow_range() function

Improve NetworkServiceTestCase._get_ip_flow_range() function

Improve the method to calculate the CIDR IP address range (first one,
last one). IPv4Network or IPv6Network hosts() is an iterator throught
the list of valid IPs. If the network is too big, the generation of a
list [1] can be very expensive.

[1]https://github.com/opnfv/yardstick/blob/master/yardstick/benchmark/scenarios/networking/vnf_generic.py#L90

JIRA: YARDSTICK-1339

Change-Id: I9352b2e7ff4925c246df8b40ecf43e8dc96c42d2
Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>