JCLOUDS-774 inconsistent region and zone mappings.

It appears that the "jclouds.regions" whitelist is being ignored in ec2. This can cause the zone supply to come back as null. This fixes that.

https://issues.apache.org/jira/browse/JCLOUDS-774
This works for me. I wonder if this is slightly more efficient, since it avoids transforming values that will not be returned:
```
Map<String, URI> regionToUris = api.getAvailabilityZoneAndRegionApi().get().describeRegions();
if (!whitelistedRegionIds.isEmpty()) {
   regionToUris = Maps.filterKeys(regionToUris, Predicates.in(whitelistedRegionIds));
}
return Maps.transformValues(regionToUris, Suppliers2.<URI>ofInstanceFunction());
```
But also fine with leaving as-is if you prefer.