Ensure pid file is removed when metadata ns daemon receives SIGTERM

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