Add confd version to YANG + .cfg files

[troglobit]

To facilitate upgrading .cfg format we need to include a version field in YANG. For example, we want to change our infix-system:motd from type string to type binary.

I propose using the confd version in configure.ac and including it in infix-system-software.yang or infix-system.yang.

[mattiaswal]

Relates to #178