Add AttachedDisk class, related configurations and tests

[mziccard]

This PR adds support for Compute's instances attached disks.

Attached disks are provided to an instance upon creation or via the attachDisk API. Since attachDisk only supports attaching already existing persistent disks (no support for scratch disks or creation on the fly) our attachDisk method (coming in a following PR) will look like:

Operation attachDisk(InstanceId instance, PersistentDiskConfiguration configuration,
    OperationOption... options);

Operation attachDisk(InstanceId instance, String deviceName,
    PersistentDiskConfiguration configuration, OperationOption... options);

Operation attachDisk(InstanceId instance, String deviceName,
    PersistentDiskConfiguration configuration, int deviceIndex,
    OperationOption... options);

Also the InstanceInfo.of method will look like:

InstanceInfo of(InstanceId instance, MachineTypeId machineType,
    AttachedDisk attachedDisk, NetworkInterface networkinterface);

However an instance can be created only if an AttachedDisk with PersistentDiskConfiguration or CreateDiskConfiguration is passed, and this method does not enforce that constraint.

[coveralls]

Changes Unknown when pulling 6e93362 on mziccard:compute-attached-disk into * on GoogleCloudPlatform:compute-alpha*.

[aozarov]

I am confused.

The associated link, attached disks specify that both PERSISTENT AND SCRATCH are valid options as a disk type.

[mziccard]

I am confused.
The associated link, attached disks specify that both PERSISTENT AND SCRATCH are valid options as a disk type.

I guess that is just a copy-paste from the description of an attached disk resource. In reality if I try to pass either a scratch disk or create a persistent disk on the fly (ScratchDiskConfiguration or CreateDiskConfiguration) I get the following error:

{
 "error": {
  "errors": [
   {
    "domain": "global",
    "reason": "invalid",
    "message": "Invalid value for field 'resource.initializeParams': ''. Cannot specify initialize params during attach disk, the persistent disk must exist."
   }
  ],
  "code": 400,
  "message": "Invalid value for field 'resource.initializeParams': ''. Cannot specify initialize params during attach disk, the persistent disk must exist."
 }
}

Which made me conclude that only attaching an existing persistent disk is supported.

[coveralls]

Coverage increased (+0.2%) to 83.715% when pulling ba8b011 on mziccard:compute-attached-disk into 90df0e5 on GoogleCloudPlatform:compute-alpha.

[aozarov]

As discuessed in #928 I like this approach better. Looks good to me but I didn't look carefully at the details.