etckeeper-formula for management of the etckeeper helper

etckeeper helps keep a record of changes to /etc by turning it
into a Git repo in our case.

Note that this will store the Git DB at /etc/.git, so potentially
can increase the size of your root filesystem over time.
Add start/end commits of /etc

Run etckeeper prior to, and directly after, any salt highstate runs.

This picks up any change made outside of a salt run, plus any changes made
during it.
README update
Fix duplicate naming
Use a more tolerant package dependency pattern

Don't name 'git' as a dependency, instead use etckeeper_deps

This allows us to remove the dep on bootstrap-formula too.
Fix error in path for source template
Only create start/end commits if changes happened

git commit by default will have exit code 1 if there is nothing
to commit - after all, why create a commit if there is no change.

There is an option '--allow-empty' to git commit, but this will
create an empty commit for each and every etckeeper run. This would
complicate our commit history, so instead only commit if there are changes
reported by git status.

Using 'onlyif' here also means that we don't have no-op state changes appearing
to happen.
update README
remove unncessary git status check
Improve order:0/last doc
Merge pull request #1 from ministryofjustice/initial_dev

Initial etckeeper formula for version control of /etc.