support multiple aliases for a single region

[rbayerl]

Related to #115 and #180 I have hit an issue where only the last region alias defined is actually honored. This simple patch allows for multiple aliases in a single region, but does not set any precedence for parameter resolution. E.g. consider the following:

region_aliases:
  foo: us-east-1
  bar: us-east-1

stacks:
  foo:
    foo-stack:
      template: foo.rb
  bar:
    bar-stack:
      template: bar.rb

Parameters for both stacks will look in parameters/foo and parameters/bar. Since one cannot have duplicate stack names in a region IMO this is not a huge issue. This patch will at least allow the option for logical stack_master.yml layout and ordering for parameter files.

[et304383]

@rbayerl could you try explaining the parameters issue you described in another way? I'm not sure I understand the limitation based on what you've described in your last paragraph.

[rbayerl]

Using the above example config here's what comes from irb:

irb(main):001:0> require 'stack_master'
=> true
irb(main):002:0> config = StackMaster::Config.load!('stack_master.yml')
=> #<StackMaster::Config:0x007fa261042440 @config={"template_dir"=>"./templates", "region_aliases"=>{"foo"=>"us-east-1", "bar"=>"us-east-1"}, "stacks"=>{"foo"=>{"foo-stack"=>{"template"=>"foo.rb"}}, "bar"=>{"bar-stack"=>{"template"=>"bar.rb"}}}}, @base_dir="/Users/rbayerl/foobar", @template_dir="./templates", @stack_defaults={}, @region_aliases={"foo"=>"us-east-1", "bar"=>"us-east-1"}, @region_to_aliases={"us-east-1"=>["foo", "bar"]}, @region_defaults={}, @stacks=[#<StackMaster::StackDefinition:0x007fa261029af8 @additional_parameter_lookup_dirs=["foo", "bar"], @compiler_options={}, @notification_arns=[], @s3={}, @files=[], @template="bar.rb", @region="us-east-1", @stack_name="bar-stack", @base_dir="/Users/rbayerl/foobar", @template_dir="./templates">], @template_compilers={:rb=>:sparkle_formation, :json=>:json, :yml=>:yaml, :yaml=>:yaml}>
irb(main):003:0> config.stacks[0].additional_parameter_lookup_dirs
=> ["foo", "bar"]

Since the parameter lookup directories just creates an array of all aliases these will all be included for all stacks in the region (regardless of the alias where the stack is defined). So parameters for the foo stack will look in ['parameters/foo.yml', 'parameters/us-east-1/foo.yml', 'parameters/foo/foo.yml', 'parameters/bar/foo.yml']. Hope that's at least a little more clear.

[patrobinson]

Can you please add some tests for this?

Given the age of this PR (our fault) it could also do with a rebase too