Support for multiple templates

[jeroenrinzema]

I would like to reuse parts of my YAML configurations across different pipeline steps for different cluster environments ex.: staging, testing and production. But for example not ingress configurations.

Currently is this plausible by defining different YAML files for every environment and assigning the different files to the template parameter. But this creates a lot of code duplication and makes it harder to preform changes to the Kubernetes configurations since the whole YAML configuration has to get duplicated.

Support for multiple template/secret files could allow configurations to be reused. Multiple templates are optional and could be defined by assigning an array to the template parameter instead of a string. This makes the feature both forward and backward compatible:

# Multiple template files:
template:
  - .kube.yml
  - .kube-ingress-test.yml

# Single template file:
template: .kube.yml

Besides the template parameter could the secret_template parameter also get support for multiple templates to keep consistency across the plugin configuration.