Allow acceptance tests to run with an API key or alternate authentication endpoint

This change to libargs.go will use RAX_API_KEY as an the APIKey for authentication, and OS_AUTH_URL as an OpenStack authentication endpoint. If neither is set then the current behavior remains the same: the acceptance tests use password authentication against the default endpoint.
It might not be necessary right now, but there's two reasons I'm interested in seeing this change:

#### Convenience

I proposed a that all the Rackspace samples for supported OpenStack SDKs should support the same env variables for switching common options like username, auth url or preferred region. The best option is to support the applicable subset of environment variables used by the keystone [CLI](http://docs.openstack.org/user-guide/content/keystone_client_commands.html). That's convenient both for users evaluating multiple SDKs and CLIs and for people who contribute to or test multiple OpenStack SDKs.

In fact, you improved my original proposal:
> Gophercloud tests already do this, but use OS_ prefix not RAX_.  I believe RAX_ prefix should be reserved for those settings which are unconditionally Rackspace-specific, and not OpenStack-generic.  Use OS_ for everything which is pertinent to OpenStack.

#### Future proofing

I'm worried the behavior of SDK_PROVIDER may change in the future, both because I think some changes may make sense based on our [previous chats](https://groups.google.com/forum/#!searchin/gophercloud-dev/SDK_PROVIDER/gophercloud-dev/pDPTJ1MSwaw/scFOHU5tjCMJ), and because you've told me:
> SDK_* environment variables are deprecated with 0.2.0 as well, including SDK_PROVIDER

Even if you drop support for SDK_PROVIDER from the SDK but keep it in the tests/samples, it is not clear how you would distinguish between an overridden URL that still offers Rackspace's Identity extensions vs basic OpenStack.

For example, if you want to test against [Mimic](https://github.com/rackerlabs/mimic) you currently could run:
```
SDK_PROVIDER=http://mimic.dev ./scripts/test-all.sh
```

I'd like to tell Gophercloud to talk to Mimic the mimic endpoint "as if it was Rackspace" vs "as Keystone w/out any extensions". I think a good way to do that is for SDK_PROVIDER to select the code path, but not the authentication endpoint:
```
SDK_PROVIDER=rackspace OS_AUTH_URL=https://my-rackspace-mimic.dev ... ./scripts/test-all.sh
SDK_PROVIDER=openstack OS_AUTH_URL=https://my-rackspace-mimic.dev ... ./scripts/test-all.sh
```