Introduce OS api version 15

Also, since Ganeti 2.1 will be compatible with both 10 and 15, change
the OS_API_VERSION constant to be an OS_API_VERSIONS set, and update the
places in the code that used that constat to use something else.

In particular:
  - in the qa for now we just create a fake version 10 OS
  - in the os environment we use the highest common version
    (which means we need to pass in the os to OSEnvironment)
  - when loading an OS any common version will do

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>