Fix reloading Telegraf under systemd

[PierreF]

Required for all PRs:

• CHANGELOG.md updated
• Sign CLA (if not already signed)

PR #1252 works well for stopping Telegraf, but reloading cause Telegraf to exit:

$ dpkg -l telegraf 
ii  telegraf       0.13.1~a8334c3-0
$ ps waux|grep telegraf
telegraf 13662  0.0  0.0   4508   756 ?        Ss   11:31   0:00 /bin/sh -c /usr/bin/telegraf -config /etc/telegraf/telegraf.conf -config-directory /etc/telegraf/telegraf.d  >>/var/log/telegraf/telegraf.log 2>>/var/log/telegraf/telegraf.log
telegraf 13663  0.3  0.2 227988 23380 ?        Sl   11:31   0:00 /usr/bin/telegraf -config /etc/telegraf/telegraf.conf -config-directory /etc/telegraf/telegraf.d
$ sudo systemctl reload telegraf
$ sudo systemctl status telegraf
[...]
   Active: inactive (dead) since jeu. 2016-05-26 11:33:08 CEST; 3s ago
[...]
 Main PID: 13662 (code=killed, signal=HUP)
$ ps waux|grep telegraf
nothing

This is caused since $MAINPID (used by ExecReload=/bin/kill -HUP $MAINPID) is the sh process. When sh process get a SIGHUP, it exits which cause child process (Telegraf) to be killed.

This PR add an "exec" which cause sh processed to be replaced by Telegraf, thus $MAINPID is now Telegraf itself.

$ dpkg -l telegraf 
ii  telegraf       0.13.1~46f0196-0
$ ps waux|grep telegraf
telegraf 15816  1.0  0.2 301976 23756 ?        Ssl  11:37   0:00 /usr/bin/telegraf -config /etc/telegraf/telegraf.conf -config-directory /etc/telegraf/telegraf.d
$ sudo systemctl reload telegraf
$ sudo systemctl status telegraf
[...]
   Active: active (running) since jeu. 2016-05-26 11:37:04 CEST; 35s ago
[...]
 Main PID: 15816 (telegraf)
$ ps waux|grep telegraf
telegraf 15816  0.5  0.3 460808 30972 ?        Ssl  11:37   0:00 /usr/bin/telegraf -config /etc/telegraf/telegraf.conf -config-directory /etc/telegraf/telegraf.d

Logs continue to be written to /var/log/telegraf/telegraf.log

[sparrc]

thanks @PierreF