Adding support for disk types, multiple disks and auto_delete_boot_disk option.

This pull request adds three new options for disks:

1. Support for disk types (pd-ssd and pd-standard) to gce_disk (disk_type) and gce_instance (boot_disk_type). 
2. Possibility for multiple disks in gce_instance. 
3. auto_delete_boot_disk option for gce_instance.