Fix for parsing of existing IAM policy

[jbrook]

During provisioning I was running into an error when trying to check for an existing SNS permission policy. I have code like this to define the permission:

lambdaWelcome := sparta.NewLambda(roleDefinition, sendWelcomeEmail, welcomeOptions)

  lambdaWelcome.Permissions = append(lambdaWelcome.Permissions, sparta.SNSPermission{
    BasePermission: sparta.BasePermission{
      SourceArn: "arn:aws:sns:eu-west-1:7056640223706:development__global__user__didJoin",
    },
  })

And I was getting the following error:

DEBU[0016] Ensuring IAM Role results                     Principal=sns.amazonaws.com PrincipalActions=[sns:ConfirmSubscription sns:GetTopicAttributes sns:Subscribe sns:Unsubscribe]
panic: interface conversion: interface {} is nil, not sparta.ArbitraryJSONObject

goroutine 1 [running]:
github.com/mweagle/Sparta.ensureIAMRoleResource(0x728db0, 0x11, 0x7ac2a0, 0x4c, 0xc820112750, 0xc820076e40, 0x0, 0x0, 0x0, 0x0)
  /Users/jbrook/go-projects/src/github.com/mweagle/Sparta/provision_utils.go:107 +0x15b0
github.com/mweagle/Sparta.ensureConfiguratorLambdaResource(0x728db0, 0x11, 0x7ac2a0, 0x4c, 0xc820112750, 0x7fff5fbff72b, 0x19, 0xc82000a163, 0x17, 0xc820076e40, ...)
  /Users/jbrook/go-projects/src/github.com/mweagle/Sparta/provision_utils.go:46 +0xf8

It seems like the structure coming back for the resource in my case was rather different from what the code was expecting.

[mweagle]

@jbrook - Thanks for the writeup & PR. Writing up a test case to enforce and will merge shortly. Cheers.

[mweagle]

Looks good - locally confirmed error & PR fix with bd98a64. Merging.