Use user's local ssh config

[xiaochuanyu]

I use a different users and keys for SSH depending on which host(s). kafka-check doesn't allow this right now. This PR attempts to parse user's local ssh config and make use of it. Sample ssh config where this applies:

Host 1.2.3.4
   User ec2-user
   IdentityFile ~/.ssh/key.pem
   StrictHostKeyChecking no
   UserKnownHostsFile /dev/null

[simplesteph]

@xiaochuanyu I did something similar here: https://github.com/Yelp/kafka-utils/pull/138/files It may be worth refactoring your code and isolate the ssh client construction method for all the other components to benefit from it