providers/aws: add name_prefix option to launch config
providers/aws: add name_prefix option to launch config

See #2911.

This adds a `name_prefix` option to `aws_launch_configuration` resources.

When specified, it is used instead of `terraform-` as the prefix for the
launch configuration.  It conflicts with `name`, so existing
functionality is unchanged.  `name` still sets the name explicitly.

Added an acceptance test, and updated the site documentation.

Ninir/terraform-provider-aws,nbaztec/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws,ClementCunin/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ClementCunin/terraform-provider-aws,terraform-providers/terraform-provider-aws,ashinohara/terraform-provider-aws,terraform-providers/terraform-provider-aws,nbaztec/terraform-provider-aws,nbaztec/terraform-provider-aws,nbaztec/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,kjmkznr/terraform-provider-aws,ClementCunin/terraform-provider-aws,ashinohara/terraform-provider-aws,ClementCunin/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ashinohara/terraform-provider-aws,terraform-providers/terraform-provider-aws,ashinohara/terraform-provider-aws,Ninir/terraform-provider-aws,kjmkznr/terraform-provider-aws,charles-at-geospock/terraform-provider-aws