Add apply_parallelism input to support serial execution

[nadenf]

Use-cases

There are sometimes bugs which need serial execution i.e.

terraform apply -parallelism=1

Proposal

Provide a new input called "apply_parallelism" which allows you to set this variable per workspace.

[jweigand]

@nadenf this is already possible by creating an environment variable in your workspace - use the tfe_variable resource to create the variable, set the value to whatever you need. See https://www.terraform.io/cloud-docs/workspaces/variables#parallelism