Support parameter expansion of environment in mapping config

[isc-pbarton]

In the mapping configuration, allow a parameter that will be expanded to the environment name.
For example, I should be able to map ESD to /config/<env> - this will expand to /config/development, /config/test, or /config/live depending on the system mode.
This is necessary if using system default settings and with #248 source controlling mapping configuration so it must be the same across environments.