[pan-os] Add automation for versions retrieval (#371)

Based on https://github.com/mrjcap/panos-versions/.

Closes #370.

src/pan-os.py

@@ -0,0 +1,11 @@
+from common import dates, http, releasedata
+
+"""Fetches pan-os versions from https://github.com/mrjcap/panos-versions/."""
+
+with releasedata.ProductData("pan-os") as product_data:
+    versions = http.fetch_url("https://raw.githubusercontent.com/mrjcap/panos-versions/master/PaloAltoVersions.json").json()
+
+    for version in versions:
+        name = version['version']
+        date = dates.parse_datetime(version['released-on'])
+        product_data.declare_version(name, date)