Add support for multiple HUB's (one HUB per region)

[andyfase]

The current poller script contains several race conditions in the use case where a hub is required in different regions. If the Transit VPC stack is created in multiple regions currently then it is not possible to control (for a given VPC/VGW) which hub in which region the VPN connection will be created for.

This pull request makes modifications to the poller script to remove these race conditions by extending the use of the value of HUB_TAG beyond the default "true" setting.

With this PR the HUB_TAG value can now be:

1. HUB_TAG_VALUE (current behaviour)
2. A valid AWS region code i.e. us-east-1, us-west-2, ca-central-1 etc

If the the value of HUB_TAG is a AWS region then the poller function will ONLY operate on the found VGW is its tagged with the same AWS region that the lambda function is working on. This allows VGW's to be targeted towards specific hub's by the value of the tag.

If the VGW is tagged with HUB_TAG_VALUE then the behavior of the lambda function is unchanged.

The lambda function has been modified in the following ways

1. Function isValidRegion added to determine if a given string is a valid AWS region
2. Modification to detection of a spoke vgw to include comparison of the value of HUB_TAG against current lambda region (AWS_DEFAULT_REGION)
3. Modification to updateConfigXML function to set "create" mode if value of HUB_TAG is a valid AWS region
4. Modification in deletion detection to only start deletion loop if tag value is also not a valid AWS region
5. Added a further check in the deletion loop to only process deletion if configuration of the spoke VPN actually exists in the S3 bucket. This avoids a deletion attempt from a region which did not originally setup the spoke in the first place.

Testing performed

1. Setup of single stack in single AWS region and used default tag value true on separate VGW's across multiple regions to confirm behavior unchanged in this use-case
2. Setup of dual stack in two AWS regions and used new tag values of different region codes across multiple VGW's to confirm that only the targetted hub lambda functions pick up the connections
3. As per point 2 confirmed the same for deletions (only targeted regions process the deletions)

[hvital]

Thanks for your contribution. This PR will be evaluated for the next version and we'll update to this thread once we have more information.