Add PID file support

[cce]

Fixes #878 by allowing for systemd, init scripts, etc, to be able to better manage snapteld and send it kill or restart signals.

Summary of changes:

• Added --pid-file flag and pid_file config file option to snapteld
• Added PIDFile to snapteld Config struct.

Testing done:

• Added PIDFile to command-line flags testing in snapteld_test.go.

@intelsdi-x/snap-maintainers

[WangJialei-A]

@katarzyna-z I think it is better to handle the PID file in the service script, but not in snapteld.