terraform_module_pinned_source: support additional default_branches

[bendrucker]

Expands the list of default branches and allows users to configure additional branch names via config. Adapted from #1123.

rule "terraform_module_pinned_source" {
  enabled = true
  default_branches = ["dev"]
}

module "unpinned" {
  source = "git://hashicorp.com/consul.git?ref=dev"
}

Warning: Module source "git://hashicorp.com/consul.git?ref=dev" uses a default branch as ref (dev) (terraform_module_pinned_source)

Closes #1123