Proposal: feature flag implementation for Juju

[Tim Penhey]

Hi all,

There are often times when we want to hook up features for testing that
we don't want exposed to the general user community.

In the past we have hooked things up in master, then when the release
branch is made, we have had to go and change things there.  This is a
terrible way to do it.

Here is my proposal:

http://reviews.vapour.ws/r/531/diff/#

We have an environment variable called JUJU_FEATURE_FLAGS. It contains
comma delimited strings that are used as flags.

The value is read when the program initializes and is not mutable.

Simple checks can be used in the code:

if featureflag.Enabled("foo") {
  // do foo like things
}

Thoughts and suggestions appreciated, but I don't want to have the
bike-shedding go on too long.

Tim