feat(k8s): extend k8s provider to fetch configmap

[bersalazar]

The k8s provider now supports fetching values from ConfigMaps.

Covers: #187

[lucasfcnunes]

@bersalazar, we should've updated the following:

vals/README.md

Lines 727 to 748 in 17fd11a

	### Kubernetes
	Fetch value from a Kubernetes:
	- `ref+k8s://API_VERSION/KIND/NAMESPACE/NAME/KEY[?kubeConfigPath=<path_to_kubeconfig>&kubeContext=<kubernetes context name>]`
	Authentication to the Kubernetes cluster is done by referencing the local kubeconfig file.
	The path to the kubeconfig can be specified as a URI parameter, read from the `KUBECONFIG` environment variable or the provider will attempt to read `$HOME/.kube/config`.
	The Kubernetes context can be specified as a URI parameteter.
	Environment variables:
	- `KUBECONFIG` contains the path to the Kubeconfig that will be used to fetch the secret.
	Examples:
	- `ref+k8s://v1/Secret/mynamespace/mysecret/foo`
	- `ref+k8s://v1/Secret/mynamespace/mysecret/bar?kubeConfigPath=/home/user/kubeconfig`
	- `secretref+k8s://v1/Secret/mynamespace/mysecret/baz`
	- `secretref+k8s://v1/Secret/mynamespace/mysecret/baz?kubeContext=minikube`
	> NOTE: This provider only supports kind "Secret" in apiVersion "v1" at this time.

Specially the part:

NOTE: This provider only supports kind "Secret" in apiVersion "v1" at this time.

[lucasfcnunes]

Thanks for addressing #191 (comment) on #192