Skip the boot disk on gce during backup

On gce, the boot disk is a persistent disk and the `:create` action on `rightscale_backup` resource also backs up this boot device. Since there is no way of backing up specific devices (#2), we should at least skip the boot device.

There seems to be a way to differentiate between boot disk and a regular disk. Here is the resource_uid of a boot disk:
```
projects/rightscale.com:publish/disks/boot-i-c75edc78a-rs:projects/rightscale.com:publish/instances/i-c75edc78a
```
and here is the resource_uid of a regular persistent disk:
```
projects/rightscale.com:publish/disks/v-f4a0b7557:projects/rightscale.com:publish/instances/i-c75edc78a
```