Improve resilience of the Bamboo server and remote agents configuration file update

Problem Definition

Bamboo configuration files are updated in place when needed for both the server and remote agents. In rare cases if the update fails for reasons outside Bamboo (for example corrupted filesystem or a failed write operation) then the configuration files can be empty. This would prevent Bamboo server or agents to start.

Suggested Solution

Update the configuration files on both the Bamboo server and remote agents using a copy on write approach, i.e. create a temporary config file with the updates and then rename to the original file. If the write + rename operation succeeds then the file is updated. If any of the two fails then the original configuration, before the update, is used.
This approach ensures that in all circumstances the Bamboo server and agents will have a valid configuration file.