[Fuchsia] Disable use of SSH connection multiplexing.

Connection multiplexing results in unexpected hangs or resets when used
by our runner scripts, so disable it until the root cause is found and
resolved upstream.

Bug: 900790
Change-Id: I5025a5d652cdcbfe0c206fc4bf42366efbee07be
Reviewed-on: https://chromium-review.googlesource.com/c/1359489Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613377}

build/fuchsia/boot_data.py

@@ -12,6 +12,8 @@ import tempfile
 import time
 import uuid
 
+# TODO(https://crbug.com/900790): Re-enable multiplexing once upstream issues
+# are resolved.
 _SSH_CONFIG_TEMPLATE = """
 Host *
   CheckHostIP no
@@ -24,8 +26,8 @@ Host *
   IdentityFile {identity}
   ServerAliveInterval 2
   ServerAliveCountMax 5
-  ControlMaster auto
-  ControlPersist 1m
+  ControlMaster no
+  ControlPersist no
   ControlPath /tmp/ssh-%r@%h:%p
   ConnectTimeout 5
   """