Don't make client addressbook source a required configuration parameter.

Don't make client addressbook source a required configuration parameter.

Chicken and egg problem: when turba is first installed, and using a source like
localsql with shares enabled, there will be no existing address books found to list.
If left unpopulated, this will cause client/* api calls to fail, but before we made
this a select list, we made it default to 'localsql' - which would cause the clients/*
api methods to fail since that's not a valid source when using shares.

 So, making this non-required doesn't loose anything we had before.