added subcommand keypairs to list SSH key pairs

Example output of this subcommand (one keypair with the name vagrant
is available on Rackspace Servers):

```
$ vagrant rackspace keypairs
==> default: KeyPair Name
==> default: vagrant
```
Confirm Rackspace server destruction.

This pull request implements a fix for
mitchellh/vagrant-rackspace#114
Merge pull request #116 from dkinzer/confirm-before-destroy

Confirm Rackspace server destruction.
Merge pull request #116 from dkinzer/confirm-before-destroy

Confirm Rackspace server destruction.
Merge pull request #117 from dkinzer/fix-no-message-error

Fix uninitialized constant error.
Merge pull request #90 from berendt/subcommand_list_keys

added subcommand keypairs to list SSH key pairs