compute: add promise support

[callmehiphop]

Breaking changes ahead! ⚠️

This adds Promise support for the @google-cloud/compute module!

Summary of changes

The following methods previously returned a stream in the event that the callback parameter was omitted. This functionality has since moved to a different method entirely.

• Compute#getAddresses -> Compute#getAddressesStream
• Compute#getAutoscalers -> Compute#getAutoscalersStream
• Compute#getDisks -> Compute#getDisksStream
• Compute#getInstanceGroups -> Compute#getInstanceGroupsStream
• Compute#getFirewalls -> Compute#getFirewallsStream
• Compute#getHealthChecks -> Compute#getHealthChecksStream
• Compute#getMachineTypes -> Compute#getMachineTypesStream
• Compute#getNetworks -> Compute#getNetworksStream
• Compute#getOperations -> Compute#getOperationsStream
• Compute#getRegions -> Compute#getRegionsStream
• Compute#getRules -> Compute#getRulesStream
• Compute#getServices -> Compute#getServicesStream
• Compute#getSnapshots -> Compute#getSnapshotsStream
• Compute#getSubnetworks -> Compute#getSubnetworksStream
• Compute#getVMs -> Compute#getVMsStream
• Compute#getZones -> Compute#getZonesStream
• InstanceGroup#getVMs -> InstanceGroup#getVMsStream
• Network#getFirewalls -> Network#getFirewallsStream
• Network#getSubnetworks -> Network#getSubnetworksStream
• Region#getAddresses -> Region#getAddressesStream
• Region#getOperations -> Region#getOperationsStream
• Region#getRules -> Region#getRulesStream
• Region#getSubnetworks -> Region#getSubnetworksStream
• Zone#getAutoscalers -> Zone#getAutoscalersStream
• Zone#getDisks -> Zone#getDisksStream
• Zone#getInstanceGroups -> Zone#getInstanceGroupsStream
• Zone#getMachineTypes -> Zone#getMachineTypesStream
• Zone#getOperations -> Zone#getOperationsStream
• Zone#getVMs -> Zone#getVMsStream

Each callback accepting change has also been updated to return a Promise instance in the event that thecallback was omitted.