Fix UsersSync by retrieving global instance correctly

The UsersSync Master class does not have a base class which provides
the global() function which results in the following error when trying
to sync users from a master:

ERROR> GlobalImpl.pm:641 EBox::GlobalImpl::__ANON__ - Failed to save
changes in module users: Can't locate object method "global" via package
"EBox::UsersSync::Master" at /usr/share/perl5/EBox/UsersSync/Master.pm
line 304.

We therefore need to get the global instance via EBox::Global.