Flags not consistent in federated/utils/optimizers/optimizer_utils.py

Not a major/urgent issue, but just FYI:

In define_optimizer_flags [the optimizer flag is defined one way](https://github.com/google-research/federated/blob/de1627f53f8ece8f2d4a34792481efba975c5bbd/utils/optimizers/optimizer_utils.py#L91), while in create_optimizer_fn_from_flags [the optimizer flag is defined another way](https://github.com/google-research/federated/blob/de1627f53f8ece8f2d4a34792481efba975c5bbd/utils/optimizers/optimizer_utils.py#L255).

For example, if your prefix is '' then define_optimizer_flags defines the flag '_optimizer' while create_optimizer_fn_from_flags looks for 'optimizer'. I fixed it in my own repo by using the method [prefixed](https://github.com/google-research/federated/blob/de1627f53f8ece8f2d4a34792481efba975c5bbd/utils/optimizers/optimizer_utils.py#L252) in both define_optimizer_flags and create_optimizer_fn_from_flags.