Merge pull request #220 from gboutry/fix/groupadd-operation

Replace groupadd with usermod in prepare script

sunbeam-python/sunbeam/commands/prepare_node.py

@@ -52,7 +52,7 @@
 
 # Add $USER to the snap_daemon group supporting interaction
 # with the sunbeam clustering daemon for cluster operations.
-sudo addgroup $USER snap_daemon
+sudo usermod --append --groups snap_daemon $USER
 
 # Generate keypair and set-up prompt-less access to local machine
 [ -f $HOME/.ssh/id_rsa ] || ssh-keygen -b 4096 -f $HOME/.ssh/id_rsa -t rsa -N ""