jenkins-community-slave: verify HTTPS certificate in head() request

Fixes a warning:

/usr/local/lib/python3.7/dist-packages/urllib3/connectionpool.py:1004: InsecureRequestWarning: Unverified HTTPS request is being made to host 'build.ffh.zone'. Adding certificate verification is strongly advised. See: https://urllib3.readthedocs.io/en/latest/advanced-usage.html#ssl-warnings

contrib/ci/jenkins-community-slave/slave.py

@@ -70,7 +70,7 @@ if os.environ.get('SLAVE_SECRET') is None:
 
 def master_ready(url):
     try:
-        r = requests.head(url, verify=False, timeout=None)
+        r = requests.head(url, timeout=None)
         return r.status_code == requests.codes.ok
     except:
         return False