buildd: add instance configuration & perform compatibility check A compatibility tag will allow buildd to evolve in the future, providing an interface to declare a subsequent update as incompatible with the old one, forcing the user (or application) to clean the instance before use. It is possible that this may never increment if compatibility is forever maintained, but since it cannot be guaranteed, we define this up front with an extendable InstanceConfiguration for application-specific customization. Using class variables to allow application-specific extensions, read the instance configuration (if available) from /etc/craft-instance.conf using InstanceConfiguration. Set the compatibility tag to "buildd-base-v0" and provide guidance on how the applications should extend this tag. Depends on #25 #24