1600 allow providing default value in config (#1604)

docs/source/configuration.rst

@@ -412,6 +412,26 @@ Below is an example of how to integrate system environment variables in pygeoapi
            host: ${MY_HOST}
            port: ${MY_PORT}
 
+Multiple environment variables are supported as follows:
+
+.. code-block:: yaml
+
+   data: ${MY_HOST}:${MY_PORT}
+
+It is also possible to define a default value for a variable in case it does not exist in
+the environment using a syntax like: ``value: ${ENV_VAR:-the default}``
+
+.. code-block:: yaml
+
+   server:
+       bind:
+           host: ${MY_HOST:-localhost}
+           port: ${MY_PORT:-5000}
+   metadata:
+       identification:
+           title:
+               en: This is pygeoapi host ${MY_HOST} and port ${MY_PORT:-5000}, nice to meet you!
+
 
 Hierarchical collections
 ------------------------