deploy: Tighten SSH connectivity check

I accidentally tried to deploy a Baserock upgrade to a Fedora cloud
machine. Every SSH command that Morph ran got the following output:

    Please login as the user "fedora" rather than the user "root".

The existing implementation of check_ssh_connectivity() didn't raise
an exception, leading to confusing errors further down.

The new implementation produces this error:

    ERROR: Unexpected output from remote machine: Please login as the
    user "fedora" rather than the user "root".

Change-Id: Ida5a82b25d759167aa842194b0d833d0565b4acf

1 files changed, 5 insertions, 1 deletions

diff --git a/writeexts.py b/writeexts.py
index ab451d14..270cade9 100644
--- a/writeexts.py
+++ b/writeexts.py
@@ -605,12 +605,16 @@ class WriteExtension(cliapp.Application):
 
     def check_ssh_connectivity(self, ssh_host):
         try:
-            cliapp.ssh_runcmd(ssh_host, ['true'])
+            output = cliapp.ssh_runcmd(ssh_host, ['echo', 'test'])
         except cliapp.AppException as e:
             logging.error("Error checking SSH connectivity: %s", str(e))
             raise cliapp.AppException(
                 'Unable to SSH to %s: %s' % (ssh_host, e))
 
+        if output.strip() != 'test':
+            raise cliapp.AppException(
+                'Unexpected output from remote machine: %s' % output.strip())
+
     def is_device(self, location):
         try:
             st = os.stat(location)