helper/schema: Rename Timeout resource block to Timeouts

[catsby]

Pluralize configuration argument name to better represent that there is
one block for many timeouts. Also adds a timeouts block into the db_instance basic test, just to have them present in an acceptance test.

Example:

resource "example" "name" {
  name = "thing"

  timeout {
    create = "10m"
    update = "5m"
  }
}

Becomes:

resource "example" "name" {
  name = "thing"

  timeouts {
    create = "10m"
    update = "5m"
  }
}

This is a follow up change to the following:

• aa3677c
• provider/aws: Rename 'timeout' to 'build_timeout' for Codebuild #12503

cc @mitchellh

---

This is intended for v0.9 only, as Timeouts were not back ported to v0.8.x series

[mitchellh]

May want to consider making the key a constant in the file. :)

[ghost]

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