Fix envsubst issues

[Lagoja]

Summary

Ugh.

Fixes a few envsubst related issues in the Nginx plugin:

1. The envsubst package seemed to have a different version of envsubst than what I expected. I replaced this package with gettext to get the GNU version
2. envsubst was replacing all the nginx variables with blanks if they weren't included in the environment, which caused Drupal and PHP related examples to stop working. This update uses gawk to get a list of all the current environment variables and pass them as arguments to envsubst, which will make envsubst skip any unset variables.
3. Fixed Drupal and LEPP examples to use the plugin correctly

How was it tested?

Ran runtest for the examples, verified that the generated nginx.conf matched previous versions.