For nginx config, a new approach is implemented with perl.  This is because unrestricted use of `envsubst` on nginx config files will replace nginx variables like $host, $request_uri with an empty string.

Closes getodk#473

For nginx config, a new approach is implemented with mawk.

This is allows an equivalent to `envsubst`, but:

* no need to explicitly list all variables
* throw error on missing variables
* do not replace nginx vars like $host, $request_uri with an empty string

Closes getodk#473

For nginx config, a new approach is implemented with mawk.

This is similar to envusbst, but more ergonomic:

* no need to explicitly list all variables
* throw error on missing variables
* do not replace nginx vars like $host, $request_uri with empty strings (in contrast to envsubst when executed without an explicit variable list)

Risks:

There are a couple of changes which may break existing deployments:

* changing client-config.json.template
* requiring all substituted variable to be defined

Closes getodk#473

For nginx config, a new approach is implemented with mawk.

This is similar to envusbst, but more ergonomic:

* no need to explicitly list all variables
* throw error on missing variables
* do not replace nginx vars like $host, $request_uri with empty strings (in contrast to envsubst when executed without an explicit variable list)

Risks:

There are a couple of changes which may break existing deployments:

* changing client-config.json.template
* requiring all substituted variable to be defined

Closes getodk#473