Can't `ccm remove` clusters with installation directories that no longer exist

You can reproduce this with:

``` sh
git clone /home/mambocab/cstar_src/cassandra
cd cassandra
ant
cd ..
ccm create cant-touch-this -n 1 --install-dir cassandra
rm cassandra
rm -rf cassandra
ccm remove cant-touch-this
```

The error you get when this happens is:

```
Traceback (most recent call last):
File "/home/mambocab/miniconda/envs/cstar_te/bin/ccm", line 74, in
cmd.run()
File "/home/mambocab/miniconda/envs/cstar_te/lib/python2.7/site-packages/ccmlib/cmds/cluster_cmds.py", line 409, in run
cluster = ClusterFactory.load(self.path, self.other_cluster)
File "/home/mambocab/miniconda/envs/cstar_te/lib/python2.7/site-packages/ccmlib/cluster_factory.py", line 28, in load
if common.isDse(install_dir):
File "/home/mambocab/miniconda/envs/cstar_te/lib/python2.7/site-packages/ccmlib/common.py", line 354, in isDse
raise ArgumentError('Installation directory does not contain a bin directory: %s' % install_dir)
ccmlib.common.ArgumentError: Installation directory does not contain a bin directory: /home/mambocab/tmp/scratch-1449518495/cassandra
```

Seems silly to require the directory you're removing to be a valid C* directory when the goal is to remove it, especially since `ccm remove` can be part of the procedure to get rid of a borked install.