* Use context in aws DescribeRegionsWithContext
In the current way, DescribeRegions is used which doesn't allow
cancelling the request if the context changes. Using
DescribeRegionsWithContext is the preferred way.
* Fix context variable
* Revert GetRegionsWithContext to GetRegions
GetRegions is not an AWS SDK method. Hence, GetRegions should be enough
as the name change is not needed for context implementation.
Fix unexpected error when creating a new cloudwatch datasource.
Involves a fair amount of refactoring, so if this causes unexpected issues related to region fetching we can turn this off with the cloudwatchNewRegionsHandler feature toggle, although we do not predict it will so we are enabling it to default to true and hope to remove it shortly.
