Enum values are never validated

[cottsay]

• The dynamic_reconfigure.client.Client class currently propagates changes to enum values that are not one of the defined values for the enum.
• rosrun dynamic_reconfigure dynparam set ... allows a user to set an invalid value.
• The C++ server also accepts the invalid values, which means that users must manually check the validity of the config.

I'm not sure it would be safe to add these checks post-release, but I thought the issue should be raised.

Inspired by https://github.com/ros-visualization/rqt_reconfigure/pull/80#issuecomment-642733360