Check CLI option dependencies cleanly

[nchammas]

Option dependencies that could not be checked directly using Click are now checked using a few new utility functions. This means users get nice, fast error messages when they don't provide the right options to Flintrock, rather than getting some ugly barf from EC2 or worse--having Flintrock launch an unusable cluster.

For example, before this PR:

$ flintrock launch nick
Launching 2 instances...
Exception: Error reading SSH protocol banner
Traceback (most recent call last):
  File ".../flintrock/venv/lib/python3.5/site-packages/paramiko/transport.py", line 1724, in _check_banner
    buf = self.packetizer.readline(timeout)
  File ".../flintrock/venv/lib/python3.5/site-packages/paramiko/packet.py", line 326, in readline
    buf += self._read_timeout(timeout)
  File ".../flintrock/venv/lib/python3.5/site-packages/paramiko/packet.py", line 481, in _read_timeout
    raise EOFError()
EOFError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File ".../flintrock/venv/lib/python3.5/site-packages/paramiko/transport.py", line 1594, in run
    self._check_banner()
  File ".../flintrock/venv/lib/python3.5/site-packages/paramiko/transport.py", line 1728, in _check_banner
    raise SSHException('Error reading SSH protocol banner' + str(e))
paramiko.ssh_exception.SSHException: Error reading SSH protocol banner

Exception: Error reading SSH protocol banner[Errno 54] Connection reset by peer
Traceback (most recent call last):
  File ".../flintrock/venv/lib/python3.5/site-packages/paramiko/transport.py", line 1724, in _check_banner
    buf = self.packetizer.readline(timeout)
  File ".../flintrock/venv/lib/python3.5/site-packages/paramiko/packet.py", line 326, in readline
    buf += self._read_timeout(timeout)
  File ".../flintrock/venv/lib/python3.5/site-packages/paramiko/packet.py", line 479, in _read_timeout
    x = self.__socket.recv(128)
ConnectionResetError: [Errno 54] Connection reset by peer

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File ".../flintrock/venv/lib/python3.5/site-packages/paramiko/transport.py", line 1594, in run
    self._check_banner()
  File ".../flintrock/venv/lib/python3.5/site-packages/paramiko/transport.py", line 1728, in _check_banner
    raise SSHException('Error reading SSH protocol banner' + str(e))
paramiko.ssh_exception.SSHException: Error reading SSH protocol banner[Errno 54] Connection reset by peer

At least one node raised an error: Error reading SSH protocol banner
Do you want to terminate the 2 instances created by this operation? [Y/n]: 
Terminating instances...
At least one node raised an error: Error reading SSH protocol banner

And with this PR:

Error: Missing option "--ec2-key-name" is required by "--provider ec2".

Before this PR:

$ flintrock launch nick
EC2ResponseError: 400 Bad Request
<?xml version="1.0" encoding="UTF-8"?>
<Response><Errors><Error><Code>InvalidAMIID.Malformed</Code><Message>Invalid id: "None" (expecting "ami-...")</Message></Error></Errors><RequestID>275a403d-da32-4bce-81d7-e02f2e66fb18</RequestID></Response>

With this PR:

Error: Missing option "--ec2-ami" is required by "--provider ec2".

Fixes #5.