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.

src/util/lib.rs

@@ -15,3 +15,4 @@ pub mod sled_counter;
 pub mod time;
 pub mod token_bucket;
 pub mod tranquilizer;
+pub mod version;