update enrol/ldap to work with roles.

update enrol/ldap to work with roles.
Credit: Alastair Pharo <alastair@catalyst.net.nz>

1) Some config settings have changed (ones that related specifically to
teachers and students).  There is a check in place however to migrate
old configurations to new ones.

2) Perviously two syncs happened - one for students, one for teachers.
Now sync gets called the same number of times as there are roles.
Those roles that have config settings associated with them then run
through all the records.  This means syncing takes longer the more
roles you configure (which is expected anyway I suppose).