Add deprecation message to trafficserver.set_var function

The set_var function has a deprecation notice in the documentation
only for the trafficserver.set_var function, but no warning at the
CLI/logs. This PR adds a "warn_util" message.

Since there haven't been any warnings to the user about the removal
of this function, the version is being bumped from Oxygen to Fluorine
so users have time to adapt as needed.

salt/states/trafficserver.py

@@ -6,6 +6,9 @@ Control Apache Traffic Server
 .. versionadded:: 2015.8.0
 '''
 
+# Import Salt libs
+import salt.utils
+
 
 def __virtual__():
     '''
@@ -237,7 +240,7 @@ def set_var(name, value):
     '''
     Set Traffic Server configuration variable values.
 
-    .. deprecated:: Oxygen
+    .. deprecated:: Fluorine
         Use ``trafficserver.config`` instead.
 
     .. code-block:: yaml
@@ -254,6 +257,11 @@ def set_var(name, value):
             - value: cdn.site.domain.tld
 
     '''
+    salt.utils.warn_until(
+        'Fluorine',
+        'The \'set_var\' function has been deprecated and will be removed in Salt '
+        '{version}. Please use \'trafficserver.config\' instead.'
+    )
     return config(name, value)