Re-add the ability to set username on zuul-executor

Sometimes we need to log in to a nodepool node using a username of
something other than zuul. This used to be possible by setting that
[launcher] username= property. Re-enable it with the [executor]
default_username= property.

default_username is used instead of simply username as it is likely in
future this information will be able to be supplied by nodepool or other
in a node or image specific way. At which time that information will be
used in priority to the default specified in zuul.

Change-Id: Icf657b4f0bbe34e182307b9eea0cd64a8d813464

etc/zuul.conf-sample

@@ -18,6 +18,9 @@ git_dir=/var/lib/zuul/git
 ;git_user_name=zuul
 zuul_url=http://zuul.example.com/p
 
+[executor]
+default_username=zuul
+
 [webapp]
 listen_address=0.0.0.0
 port=8001

zuul/executor/server.py

@@ -239,6 +239,12 @@ class ExecutorServer(object):
         else:
             self.merge_root = '/var/lib/zuul/executor-git'
 
+        if self.config.has_option('executor', 'default_username'):
+            self.default_username = self.config.get('executor',
+                                                    'default_username')
+        else:
+            self.default_username = 'zuul'
+
         if self.config.has_option('merger', 'git_user_email'):
             self.merge_email = self.config.get('merger', 'git_user_email')
         else:
@@ -672,6 +678,7 @@ class AnsibleJob(object):
             ip = node.get('interface_ip')
             host_vars = dict(
                 ansible_host=ip,
+                ansible_user=self.executor_server.default_username,
                 nodepool_az=node.get('az'),
                 nodepool_provider=node.get('provider'),
                 nodepool_region=node.get('region'))