Strip not split the untrusted_wrapper

If we split the string then we end up with a list, which will never work
when looking up a driver. I assume this was supposed to have been strip
initially.

Change-Id: Ib6b3990743981853c8369ee6db0ef606fca3a5f0
Signed-off-by: Jamie Lennox <jamielennox@gmail.com>
diff --git a/zuul/executor/server.py b/zuul/executor/server.py
index f71bb92..03169b7 100644
--- a/zuul/executor/server.py
+++ b/zuul/executor/server.py
@@ -395,7 +395,7 @@
 
         if self.config.has_option('executor', 'untrusted_wrapper'):
             untrusted_wrapper_name = self.config.get(
-                'executor', 'untrusted_wrapper').split()
+                'executor', 'untrusted_wrapper').strip()
         else:
             untrusted_wrapper_name = 'bubblewrap'
         self.untrusted_wrapper = connections.drivers[untrusted_wrapper_name]