Connection names for legacy configs

It has been set like that:
connections = {'_legacy_gerrit': Connection(name='gerrit')}

This leads to that in merger.py it creates ".ssh_wrapper_gerrit" file,
basing it on key from connections dict,
but tries to use ".ssh_wrapper__legacy_gerrit", basing on connection name.

I propose to set it to "gerrit" everywhere (and the same for smtp).

Change-Id: I2c2d8bee7bcf98c5f12809f77b34e206d07b8438
diff --git a/zuul/lib/connections.py b/zuul/lib/connections.py
index cb26ba5..92ddb0f 100644
--- a/zuul/lib/connections.py
+++ b/zuul/lib/connections.py
@@ -56,11 +56,11 @@
     if 'gerrit' in config.sections():
         connections['gerrit'] = \
             zuul.connection.gerrit.GerritConnection(
-                '_legacy_gerrit', dict(config.items('gerrit')))
+                'gerrit', dict(config.items('gerrit')))
 
     if 'smtp' in config.sections():
         connections['smtp'] = \
             zuul.connection.smtp.SMTPConnection(
-                '_legacy_smtp', dict(config.items('smtp')))
+                'smtp', dict(config.items('smtp')))
 
     return connections