Play a trick with CLOEXEC to pass the envoy bootstrap configuration as
an open file descriptor to the exec'd envoy process. The file only
briefly touches disk before being unlinked.
We convince envoy to read from this open file descriptor by using the
/dev/fd/$FDNUMBER mechanism to read the open file descriptor as a file.
Because the filename no longer has an extension envoy's sniffing logic
falls back on JSON instead of YAML, so the bootstrap configuration must
be generated as JSON instead.
