Add VM name validation

[darkowlzz]

Adds VM name validation using IsDNS1123Subdomain from k8s api-machinery
validator package.

Updates all the e2e tests to replace "_" in the VM names with "-". Underscore
in the name is not a valid DNS subdomain name.

$ sudo ./bin/ignite create weaveworks/ignite-ubuntu --name my_vm
FATA[0000] metadata.name: Invalid value: "my_vm": a DNS-1123 subdomain must consist of lower case alphanumeric characters, '-' or '.', and must start and end with an alphanumeric character (e.g. 'example.com', regex used for validation is '[a-z0-9]([-a-z0-9]*[a-z0-9])?(\.[a-z0-9]([-a-z0-9]*[a-z0-9])?)*')

Fixes #622