Ensure pid file is removed when metadata ns daemon receives SIGTERM

These files from the metadata namespace proxy are not being removed
because delete_pid() is registered with atexit. This means it only runs
when a process exits normally and won't run when a process receives a
signal.

This patch registers a signal handler for SIGTERM that calls exit()
to make the process exit normally so delete_pid() gets called.

Fixes bug: 1223250

Change-Id: I6309802e2109359560ccc084559ec8e4d310cce2
Merge "Ensure pid file is removed when metadata ns daemon receives SIGTERM"