fix for user/group mismatch in recipes/server_ec2.rb

mariadb::server will install with user/group owner of 'mysql' by default, while recipe/server_ec2.rb expects  user & group to be 'mariadb'.

this patch allows recipes/server_ec2.rb to use two new default options set in attributes/server.rb:

default['mariadb']['service']['user']                  = 'mysql'
default['mariadb']['service']['group']                = 'mysql'

recipes/server.rb should also be refactored to support custom user/group.