Currently, the server defaults to looking for extension configuration files in 
/etc/mongo/extensions (see getExtensionConfDir()) .

Atlas implemented a workaround to make this file path writeable. However, this doesn't hold true on-premise, as /etc/ is not a writeable file path. Instead, we'll need to provide a configuration parameter to the server at startup time to specify where extension configuration files are expected to be found.
As part of this ticket, introduce the parameter extensionsConfigPath. For starters, we should default the parameter to /etc/mongo/extensions so as to not break Atlas deployments. We may want to consider removing this default parameter and throwing an error if no path is provided and extensions are loaded with --loadExtensions.