[__sysctl] add alpine support

cdist/conf/type/__sysctl/manifest

@@ -2,6 +2,7 @@
 #
 # 2014 Steven Armstrong (steven-cdist at armstrong.cc)
 # 2018 Takashi Yoshi (takashi at yoshi.email)
+# 2019 Nico Schottelius (nico-cdist at schottelius.org)
 #
 # This file is part of cdist.
 #
@@ -24,7 +25,7 @@ os=$(cat "$__global/explorer/os")
 
 case "$os" in
    # Linux
-   redhat|centos|ubuntu|debian|devuan|archlinux|coreos)
+   alpine|redhat|centos|ubuntu|debian|devuan|archlinux|coreos)
       :
    ;;
    # BSD

docs/changelog

@@ -4,6 +4,7 @@ Changelog
 next:
 	* Type __letsencrypt_cert: Fix beowulf's spelling (Mondi Ravi)
 	* Core: Add preos functionality (Darko Poljak)
+	* Type __sysctl: Add alpine support (Nico Schottelius)
 
 5.1.3: 2019-08-30
 	* Build: Overcome bash CDPATH when building docs (Dmitry Bogatov)