[google|compute] disk model: get_object fix for Ruby 1.8
[google|compute] disk model: Add auto_delete option to get_as_boot_disk
Merge branch 'gce_autodelete_disk_option' into rs_1.20.0
Merge branch 'gce_launch_instance_ruby18' into rs_1.20.0

Conflicts:
	lib/fog/google/models/compute/disk.rb
[google] Google compute auth needs additional scope to insert images
(cherry picked from commit e168fb28e87d79f213f0b5a61d54f4bfc9867345)