Revert "Removed set_loaded() method from Resource class"

This patch broke python-novaclient.
It turns out novaclient uses get() to support lazy loading details.
Add a docstring to get() to clarify its usage.

see novaclient/tests/v3/test_servers.py#L54
This reverts commit 0c4d2c76a6876dd8873cca2736ca5a7f3da66c69.

Change-Id: I0a2936b00de3ca17f0e40e3d5226ecdaf1329295