This attempts to decouple password-related functions, which previously
existed both in the Store and User structs, by splitting them out into a
separate interface, PasswordStore. This means that they can be more
easily swapped out during testing.
This also changes the relevant tests to use mock password functions
instead of the bcrypt-backed implementations; as a result, the tests are
much faster.
Before:
```
github.com/coreos/etcd/etcdserver/auth 31.495s
github.com/coreos/etcd/etcdserver/etcdhttp 91.205s
```
After:
```
github.com/coreos/etcd/etcdserver/auth 1.207s
github.com/coreos/etcd/etcdserver/etcdhttp 1.207s
```
