When looking for a cluster, spark-ec2 currently pulls down [info for all instances](https://github.com/apache/spark/blob/eb48fd6e9d55fb034c00e61374bb9c2a86a82fb8/ec2/spark_ec2.py#L620) and filters locally. When working on an AWS account with hundreds of active instances, this step alone can take over 10 seconds.

This PR improves how spark-ec2 searches for clusters by pushing the filter up to EC2.

Basically, the problem (and solution) look like this:

```python
>>> timeit.timeit('blah = conn.get_all_reservations()', setup='from __main__ import conn', number=10)
116.96390509605408
>>> timeit.timeit('blah = conn.get_all_reservations(filters={"instance.group-name": ["my-cluster-master"]})', setup='from __main__ import conn', number=10)
4.629754066467285
```

Translated to a user-visible action, this looks like (against an AWS account with ~200 active instances):

```shell
# master
$ python -m timeit -n 3 --setup 'import subprocess' 'subprocess.call("./spark-ec2 get-master my-cluster --region us-west-2", shell=True)'
...
3 loops, best of 3: 9.83 sec per loop

# this PR
$ python -m timeit -n 3 --setup 'import subprocess' 'subprocess.call("./spark-ec2 get-master my-cluster --region us-west-2", shell=True)'
...
3 loops, best of 3: 1.47 sec per loop
```

This PR also refactors `get_existing_cluster()` to make it, I hope, simpler.

Finally, this PR fixes some minor grammar issues related to printing status to the user. 🎩 👏

Author: Nicholas Chammas <nicholas.chammas@gmail.com>

Closes #4922 from nchammas/get-existing-cluster-faster and squashes the following commits:

18802f1 [Nicholas Chammas] ignore shutting-down
f2a5b9f [Nicholas Chammas] fix grammar
d96a489 [Nicholas Chammas] push group filter up to EC2