restapi: RSDL Missing 'fence_type' param - #969797

For .../api/hosts/xxx/fence - fence_type is a mandatory parameter.
Added this information to rsdl_metadata.yaml file.

Bug-Url: http://bugzilla.redhat.com/969797
Change-Id: I2ee34a0698e9c6df9ae07b2c0665bacac86da845
Signed-off-by: Ori Liel <oliel@redhat.com>