Fix bugs in RoleServlet

Fredy found several issues in RoleServlet.java. We need to fix it.

1. `RoleServlet` at L62 needs to use either `try` resource or put it in the `finally` block to avoid resource leak
2. `if (conf.getBoolean(ServerConfig.SENTRY_WEB_ENABLE, ServerConfig.SENTRY_WEB_ENABLE_DEFAULT))` may return an NPE if `conf.getBoolean` returns a `null` `Boolean` (not the primitive type)
3. `RoleServlet` at L45, use `Preconditions.checkNotNull` or `Objects.requreNonNull` instead of `assert`
4. `RoleServlet` at L55: `String json = new Gson().toJson(roleMap);`, it's better to reuse the `Gson` instance.