[services] Tunnelbroker - Change ConfigManager.load() to use config file path as a parameter

Summary:
To change the config file path for dev-mode introducing changes to the ConfigManager.load() method to use config file path as a parameter instead of the config file path inside the method itself.

Linear task: ENG-1101

Test Plan: Run yarn run-tunnelbroker-service service successfuly built and load the default config file.

Reviewers: karol-bisztyga, palys-swm

Reviewed By: karol-bisztyga, palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4129