Add labels to VMs

[darkowlzz]

Adds a new flag --label to VM create command for setting label on the VM
object. The flag is of StringArray type and can be used multiple times
to set more than one label.

# ignite create weaveworks/ignite-ubuntu --name my-vm1 -l label1=val1 -l label2=val2

vm inspect result:

# ignite inspect vm my-vm1
{
  "kind": "VM",
  "apiVersion": "ignite.weave.works/v1alpha2",
  "metadata": {
    "name": "my-vm1",
    "uid": "70d265ddaaa92462",
    "created": "2020-02-02T12:37:21Z",
    "labels": {
      "label1": "val1",
      "label2": "val2"
    }
...

[chanwit]

Thank you for submitting this PR, @darkowlzz :-)
Not sure if you think my comment to change Split to SplitN is making sense?

Other than this small nit, it LGTM

[chanwit]

should we use SplitN(label, "=", 2), wdyt?
if so, the value could also be possible to contain other = chars too.

[darkowlzz]

Yes, that's a good idea. I checked what happens if I pass "k1=v1,k2=v2" as label to docker, and it splits at first separator and accepts the rest of the string as the value.

            "Labels": {
                "k1": "v1,k2=v2"
            }

Updating the test to not fail for this case.
Thanks!

[darkowlzz]

Added another check to ensure that the label name isn't empty. Updated the test with this case.

[chanwit]

LGTM

[chanwit]

Thank you @darkowlzz for this wonderful PR!!