Configure from env vars at run time

In both local developent, and production.  Also, vendor the `envsubst`
binary from https://github.com/a8m/envsubst, as it supports default
values, which the gnu envsubst (from gettext-base) does not.

services/git-bridge/.gitignore

@@ -48,3 +48,6 @@ com_crashlytics_export_strings.xml
 crashlytics.properties
 crashlytics-build.properties
 fabric.properties
+
+# Local configuration files
+conf/runtime.json