Fix regressions in cgroup implementation

Two issues noticed when testing cgroup implementation:
1. Logging is not yet configured when dropPrivs() is called, so use
   print() instead of self.log.info().
2. Environment variables need to be updated when reducing privileges
   to another user. Otherwise it failes ansible check in start up
   even if the ansible versions has been installed.

Change-Id: I8f235298ec3405551feb6518ce526e6cc059c502

zuul/cmd/executor.py

@@ -87,7 +87,7 @@ class Executor(zuul.cmd.ZuulDaemonApp):
         if os.getuid() != 0:
             return
 
-        self.log.info("Changing to user %s", user)
+        print(f"Changing to user {user}")
         pw = pwd.getpwnam(user)
 
         # Change owner on our pid file so it can be removed by us after
@@ -101,6 +101,11 @@ class Executor(zuul.cmd.ZuulDaemonApp):
         os.setuid(pw.pw_uid)
         os.umask(0o022)
 
+        # Update environment to reflect the new user
+        os.environ['HOME'] = pw.pw_dir
+        os.environ['USER'] = pw.pw_name
+        os.environ['LOGNAME'] = pw.pw_name
+
     def run(self):
         self.handleCommands()