v0.13.x: Plugin enhancement - allow embedding templates within templates

[davidmccormick]

add insertTemplateFile template function which allows embedding a template file within any templatable plugin object, e.g. within in stack resource: -

        "prometheus-server-cloud-int": {
          "files": {
            "/var/run/coreos/cloud_config": {
              "content": {
                "Fn::Sub": {{ insertTemplateFile "plugins/prometheus/assets/cloud-config.yaml" $ | toJSON }}
              }
            },
            "/var/run/coreos/prometheus.yaml": {
                "content": {
                  "Fn::Sub": {{ insertTemplateFile "plugins/prometheus/assets/prometheus-config.yaml" $ | toJSON }}
              }
            },
            "/var/run/coreos/nginx_config": {
                "content": {
                  "Fn::Sub": {{ insertTemplateFile "plugins/prometheus/assets/nginx-config" $ | toJSON }}
              }
            },
            "/var/run/coreos/script_exporter.yaml": {
              "content": {
                "Fn::Sub": {{ insertTemplateFile "plugins/prometheus/assets/script-exporter-config.yaml" $ | toJSON }}
              }
            }
          }
        }

e.g. within an inserted template for userdata: -

#!/bin/bash -xe
# Trigger updates any of these files change:-
# cloud-config: {{ insertTemplateFile "plugins/prometheus/assets/cloud-config.yaml" $ | fingerprint }}
# promethus-config: {{ insertTemplateFile "plugins/prometheus/assets/prometheus-config.yaml" $ | fingerprint }}
# nginx-config: {{ insertTemplateFile "plugins/prometheus/assets/nginx-config" $ | fingerprint }}
# script-exporter-config: {{ insertTemplateFile "plugins/prometheus/assets/script-exporter-config.yaml" $ | fingerprint }}
cloud-init() {
  container_name="cloud-init-container"
...

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[codecov-io]

Codecov Report

Merging #1661 into v0.13.x will decrease coverage by 0.02%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           v0.13.x    #1661      +/-   ##
===========================================
- Coverage    25.49%   25.46%   -0.03%     
===========================================
  Files           98       98              
  Lines         5049     5054       +5     
===========================================
  Hits          1287     1287              
- Misses        3619     3624       +5     
  Partials       143      143

Impacted Files	Coverage Δ
filereader/texttemplate/texttemplate.go	5.33% <0%> (-0.39%)	⬇️

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