Allow to set the default security provider other than 'internal'

Enable to configuring default provider somewhere:
For example, /hippo:configuration/hippo:security/@hipposys:defaultsecurityprovider = 'custom1'

So, if I set a default provider somewhere to 'custom1' there, then SecurityManager should use it instead of the default value, 'internal'.
Without this improvement, it would be less convenient to change the security provider property for each user node.