fix ipamclient failures

[andrewhsu]

What this PR does / why we need it:

The ipamclient_test.go was using undefined variables.

$ go test github.com/Azure/azure-container-networking/cns/ipamclient
# github.com/Azure/azure-container-networking/cns/ipamclient
./ipamclient_test.go:30: undefined: getAddressSpacesPath
./ipamclient_test.go:31: undefined: requestPoolPath
./ipamclient_test.go:32: undefined: reserveAddrPath
./ipamclient_test.go:33: undefined: releaseAddrPath
./ipamclient_test.go:34: undefined: getPoolInfoPath
FAIL    github.com/Azure/azure-container-networking/cns/ipamclient [build failed]

This is addressed with the first commit of this PR 8123f3b

Also, ipamclient.go had a nil pointer dereference on non-200 results code responses.

$ go test github.com/Azure/azure-container-networking/cns/ipamclient
2018/06/08 04:21:25 [Listener] Started listening on localhost:42424.
2018/06/08 04:21:25 [Azure CNS] GetAddressSpace Request
2018/06/08 04:21:25 [Azure CNS] GetAddressSpace Request
2018/06/08 04:21:25 [Azure CNS] GetPoolID Request
2018/06/08 04:21:25 [Azure CNS] GetAddressSpace Request
2018/06/08 04:21:25 [Azure CNS] GetPoolID Request
2018/06/08 04:21:25 [Azure CNS] ReserveIpAddress
2018/06/08 04:21:25 [Azure CNS] ReserveIpAddress
2018/06/08 04:21:25 [Azure CNS] GetAddressSpace Request
2018/06/08 04:21:25 [Azure CNS] GetPoolID Request
2018/06/08 04:21:25 [Azure CNS] ReserveIpAddress
2018/06/08 04:21:25 [Azure CNS] ReleaseIpAddress
--- FAIL: TestReleaseIP (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
        panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x68389c]

goroutine 19 [running]:
testing.tRunner.func1(0xc42017a0d0)
        /usr/local/go/src/testing/testing.go:622 +0x29d
panic(0x6d0d60, 0x890f80)
        /usr/local/go/src/runtime/panic.go:489 +0x2cf
github.com/Azure/azure-container-networking/cns/ipamclient.(*IpamClient).ReleaseIPAddress(0xc420112030, 0xc420112b20, 0xb, 0x7293ad, 0x3, 0x0, 0x0)
        /go/src/github.com/Azure/azure-container-networking/cns/ipamclient/ipamclient.go:210 +0x6bc
github.com/Azure/azure-container-networking/cns/ipamclient.TestReleaseIP(0xc42017a0d0)
        /go/src/github.com/Azure/azure-container-networking/cns/ipamclient/ipamclient_test.go:195 +0x3d1
testing.tRunner(0xc42017a0d0, 0x73c4d0)
        /usr/local/go/src/testing/testing.go:657 +0x96
created by testing.(*T).Run
        /usr/local/go/src/testing/testing.go:697 +0x2ca
FAIL    github.com/Azure/azure-container-networking/cns/ipamclient      0.011s

This is addressed with the second commit of this PR c085007

So now the tests pass:

$ go test github.com/Azure/azure-container-networking/cns/ipamclient
ok      github.com/Azure/azure-container-networking/cns/ipamclient      0.010s

Special notes for your reviewer:

🐬

Release note:

Fix nil pointer dereference on non-200 result code responses for ipamclient ReleaseIPAddress

[msftclas]

All CLA requirements met.

[andrewhsu]

cc @abhi

[tamilmani1989]

Thank you for this PR. It looks good. Approving it.