Update anaconda variables source

Anaconda IsFinal property use to be processed out of ANACONDA_ISFINAL
environmental variable. This commit changes this behaviour to use
RELEASE_TYPE variable out of /etc/os-release instead.

Related change in Fedora spec file:
- https://src.fedoraproject.org/rpms/fedora-release/pull-request/347

Resolves: INSTALLER-4068

pyanaconda/core/product.py

@@ -80,7 +80,17 @@ def get_product_values():
     # .buildstamp, environment, stupid last ditch hardcoded defaults.
     config = configparser.ConfigParser()
     config.add_section("Main")
-    config.set("Main", "IsFinal", os.environ.get("ANACONDA_ISFINAL", "false"))
+
+    # Get IsFinal property from /etc/os-release file
+    with open('/etc/os-release') as file:
+        for line in file:
+        key, value = line.split('=')
+        if key == 'RELEASE_TYPE':
+            if value == 'release':
+                config.set("Main", "IsFinal", True))
+            else:
+                config.set("Main", "IsFinal", False))
+
     config.set("Main", "Product", os.environ.get("ANACONDA_PRODUCTNAME", "anaconda"))
     config.set("Main", "Version", os.environ.get("ANACONDA_PRODUCTVERSION", "bluesky"))