s4:provision Rework provision-backend into provision

This removes a *lot* of duplicated code and the cause of much
administrator frustration.  We now handle starting and stopping the
slapd (at least for the provision), and ensure that there is only one
'right' way to configure the OpenLDAP and Fedora DS backend

We now run OpenLDAP in 'cn=config' mode for online configuration.

To test what was the provision-backend code, a new --ldap-dryrun-mode
option has been added to provision.  It quits the provision just
before it would start the LDAP binaries

Andrew Bartlett