Warn when ignite options for a spec are present and backend is not ignite

[stealthybox]

Using the following YAML should output warnings per offending spec instead of ignoring them silently. Example:

footloose create
WARN[0000] Options under "spec.ignite" for "manager-%d" are ignored when using the "docker" backend
WARN[0000] Options under "spec.ignite" for "worker-%d" are ignored when using the "docker" backend
INFO[0000] Docker Image: quay.io/footloose/ubuntu18.04 present locally 
INFO[0000] Creating machine: test-test-0 ...
etc...

cluster:
  name: test
  privateKey: cluster-key
machines:
- count: 3
  spec:
    image: quay.io/footloose/ubuntu18.04
    name: manager-%d
    portMappings:
    - containerPort: 22
    backend: docker
    ignite:
      cpus: 4
      memory: 2GB
- count: 5
  spec:
    image: quay.io/footloose/ubuntu18.04
    name: worker-%d
    backend: docker
    ignite:
      cpus: 2
      memory: 1GB

Related #9