config: disable ssh access to a configured user account

Cloud config can now disable ssh access to non-root users.

When defining the 'users' list in cloud-configuration a boolean
'ssh_redirect_user: true' can be provided to disable ssh logins for
that user. Any ssh 'public-keys' defined in cloud meta-data will be added
and disabled in .ssh/authorized_keys. Any attempts to ssh as this user
using acceptable ssh keys will be presented with a message like the
following:

Please login as the user "ubuntu" rather than the user "youruser".

1 files changed, 9 insertions, 0 deletions

diff --git a/doc/examples/cloud-config-user-groups.txt b/doc/examples/cloud-config-user-groups.txt
index 01ecad7b..6a363b77 100644
--- a/doc/examples/cloud-config-user-groups.txt
+++ b/doc/examples/cloud-config-user-groups.txt
@@ -36,6 +36,8 @@ users:
       - <ssh pub key 1>
       - <ssh pub key 2>
   - snapuser: joe@joeuser.io
+  - name: nosshlogins
+    ssh_redirect_user: true
 
 # Valid Values:
 #   name: The user's login name
@@ -76,6 +78,13 @@ users:
 #   no_log_init: When set to true, do not initialize lastlog and faillog database.
 #   ssh_import_id: Optional. Import SSH ids
 #   ssh_authorized_keys: Optional. [list] Add keys to user's authorized keys file
+#   ssh_redirect_user: Optional. [bool] Set true to block ssh logins for cloud
+#       ssh public keys and emit a message redirecting logins to
+#       use <default_username> instead. This option only disables cloud
+#       provided public-keys. An error will be raised if ssh_authorized_keys
+#       or ssh_import_id is provided for the same user.
+# 
+#       ssh_authorized_keys.
 #   sudo: Defaults to none. Accepts a sudo rule string, a list of sudo rule
 #         strings or False to explicitly deny sudo usage. Examples:
 #