configuration: support environment variables

A common use case when running an application inside Docker is passing configuration parameters via Environment variables.

We could introduce support for reading the values of configuration parameters from environment variables in settings.json, taking inspiration from ElasticSearch’s environment variable substitution:

Environment variables referenced with the ${…} notation within the configuration file will be replaced with the value of the environment variable, for instance:

node.name:    ${HOSTNAME}
network.host: ${ES_NETWORK_HOST}

[muxator]

Implemented. The PR is #3572.

Please note that there is going to be a little Etherpad-specific remark, due to being tied to use a JSON file for configuration. Even when replacing a variable whose value would be a boolean or a number, quotes must be used. Etherpad, then, takes care of converting back the configuration parameter from string to the right type.

The documentation is in https://github.com/ether/etherpad-lite/blob/3b6b12e8482bb641252c2aa8fff225d61cd5225b/settings.json.template.

   "port":   9001,          <-- Literal values. When not using substitution,
   "minify": false              only strings must be quoted: booleans and
   "skin":   "colibris"         numbers must not.

   "port":   ${PORT}        <-- ERROR: this is not valid json
   "minify": ${MINIFY}
   "skin":   ${SKIN_NAME}

   "port":   "${PORT}"      <-- CORRECT: if you want to use a variable
   "minify": "${MINIFY}"        substitution, put quotes around its name,
   "skin":   "${SKIN_NAME}"     even if the required value is a number or a
                                boolean.
                                Etherpad will take care of rewriting it to
                                the proper type if necessary.

[muxator]

Hi, @gobengo, I am glad this enabled a new deployment possibility.

A rough breakdown of the remaining issues for the 1.8 milestone is as follows:

1. there are two remaining security issues that really have to be fixed before releasing 1.8: 1.a #3598, a problem with tar library in a transitive dependency of npm 1.b the clean-css library requires a major version bump, and this requires some work (see ~https://github.com/ether/etherpad-lite/pull/3597#issuecomment-483851384~ #3616)
2. the async migration introduced many important changes, but there are still two issues that need to be addressed (see #3568 and #3566)
3. there is some feature work that could probably be postponed to a later version

If you have the possibility of working on 1.a, 1.b or 2, this could help a lot. Thanks!