gerrit: Allow sending data with ssh command

Some commands like git-upload-pack expect data on stdin. These
commands block further execution resulting in a blocked thread. This
adds the possibility to optionally provide data for stdin of the
command.

Change-Id: I21fa2cb310a0472845d216da860e7ce06bd17825

zuul/lib/gerrit.py

@@ -184,7 +184,7 @@ class Gerrit(object):
                        key_filename=self.keyfile)
         self.client = client
 
-    def _ssh(self, command):
+    def _ssh(self, command, stdin_data=None):
         if not self.client:
             self._open()
 
@@ -195,6 +195,9 @@ class Gerrit(object):
             self._open()
             stdin, stdout, stderr = self.client.exec_command(command)
 
+        if stdin_data:
+            stdin.write(stdin_data)
+
         out = stdout.read()
         self.log.debug("SSH received stdout:\n%s" % out)