Restrict values only to allowed in cluster template

Currently Generic values are allowed for some of the parameters in baymodel/cluster template but these do not work:-
server_type is set to StingType as follows:
server_type = wsme.wsattr(wtypes.StringType(min_length=1,
                                                max_length=255),
                              default='vm')

1) It should be Enum('vm', 'bm') : vm for virtual machine and bm for baremetal.

2) Also api validation should be set for this so that user get proper error message if they tries to set server_type apart from allowed types('vm','bm').

Other info: There are other parameters like: volume_driver/network_driver which are set to StringType but they are validated at the time of creation.
So changing Type of these parameter to Enum doesn't seem to be required because with time volume_driver/network_driver can evolve to newly supported driver so api validation is enough.