feat: add multi-config and handle static ips

[sunya-ch]

• kind/feat

What this PR does / why we need it:

cherry-pick:

• 2755108#diff-fe19ec3e0abcc014e6f066867b0d49b97d44612880eeca1ed72cb43d077325ea

Which issue(s) this PR is related to:

• Fill the gap between v1.3.0 and alpha v1.2.x #256

Based on

• feat: add multi-path routes #325

Need confirm whether the following change need from f5e0550#diff-1db7710d390d807a0392e69569890b934ca93f62774731d52653cdc7f9eb012d

@@ -396,6 +396,10 @@ func loadConf(args *skel.CmdArgs) (*NetConf, string, error) {
	}
	selectResponse, err := selectNICs(n.DaemonIP, n.DaemonPort, podName, podNamespace, hostName, n.Name, nicSet, n.MasterNetAddrs)
	if err != nil {
+		if n.IPAM.Type == MultiConfigIPAMType {
+			// allow to have empty Masters
+			err = nil
+		}
		return n, deviceType, err
	}
	n.Masters = selectResponse.Masters