s4:dsdb Handle dc/domain/forest functional levels properly

s4:dsdb Handle dc/domain/forest functional levels properly

Rather than have the functional levels scattered in 4 different,
unconnected locations, the provision script now sets it, and the
rootdse module maintains it's copy only as a cached view onto the
original values.

We also use the functional level to determine if we should store AES
Kerberos keys.

Andrew Bartlett