Export more instance information in hooks

Currently we miss in hooks the instance's hypervisor, hypervisor
parameters and backend parameters. This forces hooks to query back into
ganeti, which is dangerous due to possible luxi sockets exhaustion.

This patch adds these three as INSTANCE_HYPERVISOR, INSTANCE_HV_*,
INSTANCE_BE_*. The hook environment prefixes all keys with “GANETI”, so
a default settings for a xen-pvm instance would be:

  GANETI_INSTANCE_HV_initrd_path=
  GANETI_INSTANCE_HV_kernel_args=ro
  GANETI_INSTANCE_HV_kernel_path=/boot/vmlinuz-2.6-xenU
  GANETI_INSTANCE_HV_root_path=/dev/sda1

Any dashes in parameter names are changed to underscores, since
variables with dashes are not easy to access from the shell
(alternatively we could deny those via an unittest for constants.py).

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