support defining AZs through account mapping

[Guslington]

Selecting Availability Zones

By default the vpc-v2 component will automatically select the AZs. This is achieved by looping over the max_availability_zones count and using the Fn::GetAZs Cloudformation function to select the AZ id.

max_availability_zones.times |az|
  selected_az = FnSelect(az, FnGetAZs(Ref('AWS::Region')))
end

However if you wish to define which as AZs you want to use you can by configuring a map per AWS account with the AZs you wish to use.
NOTE: the total count of AZ's defined in the map for each account has to be the same value as max_availability_zones.

To configure your AZ settings, set az_mapping to true

az_mapping: true

Then configure a map in the following structure to define your AZs

Accounts:
  '000000000000': # AWS Account Id
    AZs: '3,5,0' # Comma delimited list of numerical values that maps to the Availability Zone for that account

the numerical values will map to the Availability Zone retuned from the Fn::GetAZs function in the AWS account. For example in us-east-1 returns

[ "us-east-1a", "us-east-1b", "us-east-1c", "us-east-1d", "us-east-1e", "us-east-1f" ]

therefore the mapping AZs: '3,5,0' will use AZs [ "us-east-1d", "us-east-1f", "us-east-1a" ]

the new function to retrieve the AZ becomes

max_availability_zones.times |az|
  selected_az = FnSelect(FnSelect(az, FnSplit(',', FnFindInMap('Accounts', Ref('AWS::AccountId'), 'AZs'))), FnGetAZs(Ref('AWS::Region')))
end

[aaronwalker]

Looks good.... just a minor documentation improvement