Improve outputs and usage as a module.

This release makes two changes one of them will be breaking to current users:

simplifies output values by removing extra list element around each output
outputs for nameserver values now returned as single-level lists
non-list outputs now returned as strings
simplifies retrieval of values from other workspaces via terraform_remote_state
replaces provider declarations with required_providers block
follows recommended to keep explicit provider configurations only in the root module
allows module consumers to upgrade to newer provider versions without altering the module
eliminates need to pass aws_region and gcp_region as module inputs and declare as variables