toString() method returns enum name instead of value

A change introduced in 1.5.0 (link: https://github.com/Azure/azure-libraries-for-java/commit/1615dc2224861adc1f457744410d28274ce4cf89) incorrectly dropped the `toString()` method override for some enum classes (e.g. `VirtualMachineSizeTypes` & `IPVersion`) while correctly returning value for some other enum classes (e.g. `RollingUpgradeActionType`). As a result the affected enum classes returns the enum name (ALL_CAPS) instead of the value.