Add staticmethod on the debug task to validate targets

[beckjake]

The syntax is like this, mirroring the actual contents of a target dict in profiles.yml (and very close to the one proposed in #1754) - instead of having the adapter type a separate argument, it's part of the only parameter:

from dbt.task.debug_task import DebugTask
from dbt.exceptions import DbtConfigError

try:
  DebugTask.validate_connection({'type': 'redshift', 'host': 'abc123', ...})
except DbtConfigError as e:
  ...

I put this on the DebugTask object to re-use some code.

Added unit tests for this feature, and some very basic integration tests for dbt debug.

[drewbanin]

this LGTM but adding @cmcarthur for the official review!