userportal,webadmin: add enable SPICE proxy checkbox

Added a new checkbox under SPICE options of the console
popup window named "Enable SPICE Proxy".

The behavior:
- if the SPICE proxy is configured on system level (SpiceProxyDefault)
  + the "Enable SPICE Proxy" checkbox is enabled and checked by default
  + if the selection is changed, than it is stored in the brower level

- if the SPICE proxy is NOT configured on system level (SpiceProxyDefault):
  + the "Enable SPICE Proxy" is not editable and has a title:
    "No SPICE proxy defined on system level"
  + it is not checked
  + if it was previously checked and this preference stored on browser level,
    this preference is ignored and the checkbox will be unchecked

Change-Id: I879a4976c649f1d568f42c2d72f9e995a618d874
Bug-Url: https://bugzilla.redhat.com/924605
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>