[ipamd] throw an error on configuration validation failure

[veshij]

What type of PR is this?: Bugfix

Which issue does this PR fix: N/A

What does this PR do / Why do we need it:: Existing implementation panics in case of config validation error since returned error value is nil.

If an issue # is not available please add repro steps and logs from IPAMD/CNI showing the issue:
Launch ipamd with no ipv* env variables set:

{"level":"debug","ts":"2021-10-20T03:42:42.546Z","caller":"ipamd/ipamd.go:2164","msg":"Instance hypervisor family nitro"}
{"level":"error","ts":"2021-10-20T03:42:42.546Z","caller":"ipamd/ipamd.go:399","msg":"IPv4 and IPv6 are both disabled. One of them have to be enabled"}
{"level":"info","ts":"2021-10-20T03:42:42.546Z","caller":"aws-k8s-agent/main.go:73","msg":"Serving RPC Handler version  on 127.0.0.1:50051"}
{"level":"info","ts":"2021-10-20T03:42:42.546Z","caller":"runtime/asm_amd64.s:1371","msg":"Serving metrics on port 61678"}
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x39 pc=0x19d2a26]

goroutine 497 [running]:
github.com/aws/amazon-vpc-cni-k8s/pkg/ipamd.(*IPAMContext).StartNodeIPPoolManager(0x0)
        go/src/github.com/aws/amazon-vpc-cni-k8s/pkg/ipamd/ipamd.go:633 +0x26
created by main._main
        go/src/github.com/aws/amazon-vpc-cni-k8s/cmd/aws-k8s-agent/main.go:64 +0x32c

Testing done on this change: Y

Will this break upgrades or downgrades. Has updating a running cluster been tested?: No. No.

Does this change require updates to the CNI daemonset config files to work?: No.

Does this PR introduce any user-facing change?: No.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jayanthvn]

LGTM, Thanks for the PR.

[veshij]

Is anything needed from me to merge this PR? Not sure why codeconv check is not happy.

[veshij]

Is anything needed from me to merge this PR? Not sure why codeconv check is not happy.

cc @jayanthvn