Handled IPAM policy properly to fully support strict IPAM

[Ex4amp1e]

Issue: #614
Problem: strict IMAP has not been working for dual stack NSE
Solution: Place strict IPAM policy over group policy if it is configured by ENV

How to test
Setup NSE env

        - name: NSM_IPAM_POLICY
          value: strict
        - name: NSM_CIDR_PREFIX
          value: 172.16.1.100/27,2001:db8::/116

[denis-tingaikin]

Suggested change

	IPAMPolicy string `default:"polite" desc:"defines NSE's IPAM Policy. Possible values: polite, strict. Polite policy accepts any addresses sent by client. Strict policy resets ip_context if any of the client's addresses doesn't match endpoint's CIDR." split_words:"true"`
	IPAMPolicy newIPAMFunc `default:"polite" desc:"defines NSE's IPAM Policy. Possible values: polite, strict. Polite policy accepts any addresses sent by client. Strict policy resets ip_context if any of the client's addresses doesn't match endpoint's CIDR." split_words:"true"`

[Ex4amp1e]

Done

[denis-tingaikin]

Suggested change

	type newIPAMFunc func(...*net.IPNet) networkservice.NetworkServiceServer
	// Decode takes a string IPAM Policy and returns the IPAM Policy func
	func (f *newIPAMFunc) Decode(policy string) error {
	switch config.IPAMPolicy {
	case "strict":
	*f = func(cidrPrefix [][]*net.IPNet) {
	var ipnetList []*net.IPNet
	for _, group := range cidrPrefix {
	ipnetList = append(ipnetList, group...)
	}
	return strictipam.NewServer(func(i ...*net.IPNet) networkservice.NetworkServiceServer {
	return groupipam.NewServer(cidrPrefix)
	}, ipnetList...)
	case "polite":
	*f = func(cidrPrefix [][]*net.IPNet) {
	return groupipam.NewServer(cidrPrefix)
	}
	default:
	logrus.Fatalf("not a valid IPAM Policy: %s", config.IPAMPolicy)
	}
	}

[Ex4amp1e]

Done

[denis-tingaikin]

Suggested change

	ipnetList := []*net.IPNet{}
	for _, group := range config.CidrPrefix {
	ipnetList = append(ipnetList, group...)
	}
	var IPAMServer networkservice.NetworkServiceServer
	switch config.IPAMPolicy {
	case "strict":
	IPAMServer = strictipam.NewServer(func(i ...*net.IPNet) networkservice.NetworkServiceServer {
	return groupipam.NewServer(config.CidrPrefix)
	}, ipnetList...)
	case "polite":
	IPAMServer = groupipam.NewServer(config.CidrPrefix)
	default:
	logrus.Fatalf("not a valid IPAM Policy: %s", config.IPAMPolicy)
	}

[Ex4amp1e]

Done

[denis-tingaikin]

Suggested change

	IPAMServer,
	config.newIPAMFunc(config.CidrPrefix),

[Ex4amp1e]

Done