rest: Validation of the protocol values

We did not check protocol values provided so update of the host with
wrong protocol string failed on backend whereas it should fail during
initial validation.

Change-Id: I666b5469bcaf2bd00f793ad4320c3f37505d49b0
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>
Bug-Url: https://bugzilla.redhat.com/1170125