sysrepo: Implement parts of ietf-system module

Implement announcing OS name and OS release through ietf-system YANG
model [1], specifically via its top-level container system-state.
The properties are obtained from /etc/os-release file.

[1] https://tools.ietf.org/html/rfc7317

Change-Id: I82d5fd54659ea365232a3a9455dd73f84b8fd0d1
diff --git a/tests/system/missing-keys b/tests/system/missing-keys
new file mode 100644
index 0000000..ee45a08
--- /dev/null
+++ b/tests/system/missing-keys
@@ -0,0 +1,5 @@
+# asd
+# asd
+PRETTY_NAME="Czech Light v4-105-g8294175-dirty"
+
+# asd