Merge pull request #24 from andylyall/develop

Develop
Filtering server products based on the region context.
Setting VM IP addresses.
Allow for null snapshot source disks

Google SDK docs say:
"The source disk used to create this snapshot. Once the source disk
 has been deleted from the system, this field will be cleared, and
will not be set even if a disk with the same name has been re-created."
Check for null source image in toVirtualMachine()
Only return data centers in the requested region

Copied the cache handling strategy from AWS driver
Fix Firewall providerVlanId
Don't set firewall ports for ICMP rules

Else GCE complains
Compute instances were cached across accounts

It was not possible to have two GCE accounts loaded due to the cache
conflicts.
Merge pull request #29 from labisso/rollup-pr

Rollup PR against develop
Merge pull request #31 from labisso/develop-firewall-icmp-rules

Don't set firewall ports for ICMP rules - develop
Merge pull request #33 from labisso/develop-compute-cache

Compute instances were cached across accounts - develop
Fixed merge