Set correct auth_mechanism for updateUser

Currently the mongodb command `updateUser` defaults to SCRAM-SHA-256 but you can't update these passwords.

And also show an error when the update goes wrong.

Author: JvGinkel

lib/puppet/provider/mongodb_user/mongodb.rb

@@ -101,10 +101,12 @@ def password_hash=(_value)
       command = {
         updateUser: @resource[:username],
         pwd: @resource[:password_hash],
-        digestPassword: false
+        digestPassword: false,
+        mechanisms: @resource[:auth_mechanism] == :scram_sha_1 ? ['SCRAM-SHA-1'] : ['SCRAM-SHA-256'],
       }
 
-      mongo_eval("db.runCommand(#{command.to_json})", @resource[:database])
+      out = JSON.parse(mongo_eval("db.runCommand(#{command.to_json})", @resource[:database]))
+      raise "Failed update User password for user '#{@resource[:username]}'\n#{out}" if out['ok'] == 0
     else
       Puppet.warning 'User password operations are available only from master host'
     end