Combine node specific labels with common labels.

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### Feature Request

I explained the problem on [StackOverflow](https://stackoverflow.com/questions/64720193/jcasc-is-there-a-way-to-combine-node-specific-labels-with-common-labels), but I'll repeat it here:

Below is an example of what our current configuration looks like:

```
x-node-defaults: &node-defaults
labelString: "common-label-1 common-label-2 common-label-3"

jenkins:
nodes:
- permanent:
name: "node-1"
<<: *node-defaults
- permanent:
name: "node-2"
<<: *node-defaults
labelString: "common-label-1 common-label-2 common-label-3 node-specific-1"
```

Our actual configuration has a lot more nodes and a lot more properties configured, but this shows the problem we are having: ___We can't add node specific labels without duplicating all the common labels.___

I haven't been able to find a YAML way of concatenating strings, and while you can't exactly merge arrays in YAML, you almost can: You can make arrays of arrays. To get from an array of arrays to a label string, all you have to do is flatten the array and join the elements with space as separator. Here's an example from [StackOverflow](https://stackoverflow.com/a/57209078/310121):

```
.pip_git: &pip_git
- git config --global url."https://gitlab-ci-token:${CI_JOB_TOKEN}@gitlab.com".insteadOf "ssh://git@gitlab.com"
- mkdir -p ~/.ssh
- chmod 700 ~/.ssh
- echo "$SSH_KNOWN_HOSTS" > ~/.ssh/known_hosts
- chmod 644 ~/.ssh/known_hosts

test:
image: python:3.7.3
stage: test
script:
- *pip_git
- pip install -q -r requirements_test.txt
- python -m unittest discover tests
```

This makes `script` an array of arrays:

```
[["git config --global url.\"https://gitlab-ci-token:${CI_JOB_TOKEN}@gitlab.com\".insteadOf \"ssh://git@gitlab.com\"",
"mkdir -p ~/.ssh",
"chmod 700 ~/.ssh",
"echo \"$SSH_KNOWN_HOSTS\" > ~/.ssh/known_hosts",
"chmod 644 ~/.ssh/known_hosts"],
"pip install -q -r requirements_test.txt",
"python -m unittest discover tests"]
```

I was thinking that in addition to `labelString`, we could have a `labels` property which is an array of strings. Internally, we will flatten, join and merge it with the `labelString`. That way, it will be backwards compatible and easy to use.

I'm not too familiar with the architecture of JCasC, so I don't know how feasible my proposal is, or where it should be implemented. The implementation itself should be trivial, just need to know where to put it.