slapos/recipe/slapconfiguration: Support key.subkey.subkey like slap partition parameters

Currently, slap partition parameters only support simple key value pair like parameters and json to provide multi level keys. This patch allow user to give subkeys seprated by '.' while supporting the existing ways.

For example:

An instance parameter like this

"zodb-zeo.tcpv4-port": 8888

will be available as:

slapparameter["zodb-zeo"]["tcpv4-port"]