Addressng chsh fail due to /etc/passwd permissions

Since maas containers are running as non-privileged,
the maas root user might not be able to get lock on
the host's /etc/paasword to change shell for the maas
user. Added retry/sleep to eventually get the lock and
make the shell update for the user.
Also, using usermod instead of chsh.

Change-Id: Iaba6276bd6d54b22980aef8c6f7c5c113d0f2fff

charts/maas/templates/bin/_start.sh.tpl

@@ -36,6 +36,21 @@ then
   chmod 600 ~maas/.ssh/*
 fi
 
-chsh -s /bin/bash maas
-
+set +e
+sh_set=false
+for (( c=0; c<=10; c++ )); do
+  if chsh -s /bin/bash maas; then
+    sh_set=true
+    break
+  elif usermod -s /bin/bash maas; then
+    sh_set=true
+    break
+  else
+    sleep 2
+  fi
+done
+if [[ $sh_set = false ]]; then
+  exit 1
+fi
+set -e
 exec /sbin/init --log-target=console 3>&1