Add sweeper for SSH keys.

[andrewsomething]

This adds a sweeper for SSH keys. It also removes a custom helper for generating keys for use in tests. The SDK already provides a helper for this that we use elsewhere. There is no need to duplicate that functionality.

$ make testacc PKG_NAME=digitalocean/sshkey 
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test -v ./digitalocean/sshkey/...  -timeout 120m -parallel=5
=== RUN   TestAccDataSourceDigitalOceanSSHKey_Basic
=== PAUSE TestAccDataSourceDigitalOceanSSHKey_Basic
=== RUN   TestAccDataSourceDigitalOceanSSHKeys_Basic
=== PAUSE TestAccDataSourceDigitalOceanSSHKeys_Basic
=== RUN   TestAccDigitalOceanSSHKey_importBasic
=== PAUSE TestAccDigitalOceanSSHKey_importBasic
=== RUN   TestAccDigitalOceanSSHKey_Basic
=== PAUSE TestAccDigitalOceanSSHKey_Basic
=== CONT  TestAccDataSourceDigitalOceanSSHKey_Basic
=== CONT  TestAccDigitalOceanSSHKey_importBasic
=== CONT  TestAccDigitalOceanSSHKey_Basic
=== CONT  TestAccDataSourceDigitalOceanSSHKeys_Basic
--- PASS: TestAccDigitalOceanSSHKey_Basic (1.85s)
--- PASS: TestAccDigitalOceanSSHKey_importBasic (2.15s)
--- PASS: TestAccDataSourceDigitalOceanSSHKey_Basic (3.36s)
--- PASS: TestAccDataSourceDigitalOceanSSHKeys_Basic (5.21s)
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/digitalocean/sshkey     5.272s

[danaelhe]

looks good 👍