core, webadmin, restapi: Fix VM creation with blank template

* This change modifies how the OS and the display protocol is selected
  when a VM is created from the REST API. If the API user does not
  select a OS and the blank template is chosen, the default OS is
  obtained by the OsRepository based on the architecture of the
  selected cluster. In cases where the display protocol is omitted and
  the blank template is used, a proper default option is retrieved from
  the OsRepository.
* The New VM/New Pool dialog was changed to select the proper default
  OS according to the architecture of the selected cluster.
* This change avoids the selection of the x86_64 specific "Other OS"
  when creating a VM of another architecture.
* A map associating each architecture with their default OS was created
  in the OsRepository.

Change-Id: I880e5940c7244476e477e098445a47210ea08b5d
Signed-off-by: Vitor de Lima <vitor.lima@eldorado.org.br>
Signed-off-by: Leonardo Bianconi <leonardo.bianconi@eldorado.org.br>