Support loading a configuration file for sanlock plugin

Support loading a configuration file for sanlock plugin

Introduce a configuration file with a single parameter
'require_lease_for_disks', which is used to decide whether
it is allowed to start a guest which has read/write disks,
but without any leases.

* libvirt.spec.in: Add sanlock config file and augeas
  lens
* src/Makefile.am: Install sanlock config file and
  augeas lens
* src/locking/libvirt_sanlock.aug: Augeas master lens
* src/locking/test_libvirt_sanlock.aug: Augeas test file
* src/locking/sanlock.conf: Example sanlock config
* src/locking/lock_driver_sanlock.c: Wire up loading
  of configuration file