pkg/meta/bandwidth: increase IfbDeviceName size

* Increase entroy from 2 bytes to 7 bytes to prevent collisions
* Extract common library function for hash with prefix
* Refactor portmap plugin to use library function

fixes #347

Co-authored-by: Cameron Moreau <cmoreau@pivotal.io>
Co-authored-by: Mikael Manukyan <mmanukyan@pivotal.io>

plugins/meta/bandwidth/bandwidth_linux_test.go

@@ -105,7 +105,7 @@ var _ = Describe("bandwidth test", func() {
 		hostIP = net.IP{169, 254, 0, 1}
 		containerIP = net.IP{10, 254, 0, 1}
 		hostIfaceMTU = 1024
-		ifbDeviceName = "5b6c"
+		ifbDeviceName = "bwpa8eda89404b7"
 
 		createVeth(hostNs.Path(), hostIfname, containerNs.Path(), containerIfname, hostIP, containerIP, hostIfaceMTU)
 	})