Configure structopt to report the right version

By default, structopt reports the value provided by
the env var CARGO_PKG_VERSION, feeded by Cargo when reading
Cargo.toml. However for Garage we use a versioning based on git,
so we often report a version that is behind the real version.
In this commit, we create garage_util::version::garage() that
reports the right version and configure all structopt subcommands
to call this function instead of using the env var.

Cargo.lock

@@ -1079,7 +1079,6 @@ dependencies = [
  "futures-util",
  "garage_util 0.7.0",
  "gethostname",
- "git-version",
  "hex",
  "hyper",
  "k8s-openapi",
@@ -1177,6 +1176,7 @@ dependencies = [
  "chrono",
  "err-derive 0.3.1",
  "futures",
+ "git-version",
  "hex",
  "http",
  "hyper",