Remove alarming-looking agent log message on reboot

[cbgbt]

Issue or Feature Request:
When the brupop agent reboots a node, it allows the reboot to send a signal to the process, terminating it. This results in a log message that looks like this:

thread 'main' panicked at 'index out of bounds: the len is 1 but the index is 1', agent/src/apiclient.rs:112:56

We should have the brupop agent just std::process::exit() gracefully rather than allowing it to be killed.