Update Subnets on LoadBalancer

[SunithaGudisagarIBM]

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Relates OR Closes #0000

Output from acceptance testing:

 $ make testacc TEST=./ibm/service/vpc TESTARGS='-run=TestAccIBMISLB_basic_subnet'

resource "ibm_is_vpc" "testacc_vpc" {
  name = "sunitha-vpc"
}

resource "ibm_is_subnet" "testacc_subnet" {
  name            = "sunitha-subnet"
  vpc             = ibm_is_vpc.testacc_vpc.id
  zone            = "us-south-1"
  ipv4_cidr_block = "10.240.0.0/24"
}

resource "ibm_is_subnet" "testacc_subnet_1" {
	name = "sunitha-subnet-1"
	vpc = ibm_is_vpc.testacc_vpc.id
	zone = "us-south-2"
	ipv4_cidr_block = "10.240.64.0/24"
}
resource "ibm_is_lb" "testacc_LB" {
  name    = "sunitha-load-testing"
  subnets = [ibm_is_subnet.testacc_subnet.id]
}

data "ibm_is_lbs" "ds_lb" {
#   name = "sunitha-load-testing"
}

data "ibm_is_lb" "ds_lb" {
  name = "sunitha-load-testing"
}

[SunithaGudisagarIBM]

@hkantare can you please review this PR, our team internally reviewed ibm-vpc#168 PR.

[SunithaGudisagarIBM]