etcdmain: format usage

Fix #1649 

Follow the go cmd style and our doc at https://github.com/coreos/etcd/blob/master/Documentation/0.5/configuration.md

do we still want to print out the deprecated flags? 

/cc @bcwaldon @jonboulle @robszumski 
from go build -h
```
	-a
		force rebuilding of packages that are already up-to-date.
```