webadmin: remove storage - gray out hosts list

Host list is irrelevant for ISO/Export domains when 'format'
is unchecked (since no operation is performed by the host).
Hence, graying out the select-box.

Change-Id: I27496ae060d0255d2e8237fde3a80737ac9ff6bd
Bug-Url: https://bugzilla.redhat.com/1003241
Signed-off-by: Daniel Erez <derez@redhat.com>