Make deployment manifest error messages consistent

Signed-off-by: Adam Stegman <astegman@pivotallabs.com>
Extract file validation into a new package

Signed-off-by: Adam Stegman <astegman@pivotallabs.com>
Add deploy command

* Validate existence of CPI release given as argument

Signed-off-by: Adam Stegman <astegman@pivotallabs.com>
Fix variable name

Signed-off-by: Adam Stegman <astegman@pivotallabs.com>
Validate release given to deploy command

* Validates release as a BOSH release
* Does not validate that the release is a valid CPI

Signed-off-by: Adam Stegman <astegman@pivotallabs.com>
Signed-off-by: Phan Le <ple@pivotallabs.com>