PR 141: Handle buildpack positions

Handle positions < 1 during create

Positions cannot be < 1 on updates
Positions need to be shifted on delete
Use create in test cases to set positions properly

Move buildpack update to from controller to model

Force position = 1 for the first buildpack

Fix integration tests based on correct positioning

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>
Signed-off-by: Michael Maximilien <maxim@us.ibm.com>
Extract intent revealing variables and improve spec description

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>