Add support for scripts in the inject builder

Sometimes we want to inject environment variables from
different sources, so it's very useful to be able to use
EnvInject's script parameters. This change adds support.

Change-Id: I5b3054c0fa8c5ba45fb3f73ed50a1da52f60fb8e

tests/builders/fixtures/inject.xml

@@ -5,6 +5,8 @@
       <info>
         <propertiesFilePath>example.prop</propertiesFilePath>
         <propertiesContent>EXAMPLE=foo-bar</propertiesContent>
+        <scriptFilePath>script.sh</scriptFilePath>
+        <scriptContent>script content</scriptContent>
       </info>
     </EnvInjectBuilder>
   </builders>

tests/builders/fixtures/inject.yaml

@@ -2,3 +2,5 @@ builders:
   - inject:
       properties-file: example.prop
       properties-content: EXAMPLE=foo-bar
+      script-file: script.sh
+      script-content: script content