openstack-full: fatal_error if DKMS fails

Up to now, the dkms build for the target kernel failed but didn't stopped the build process.
As a result, openvswitch wasn't functional as expected.

This patch insure that we'll get a fatal_error if the DKMS build failed.

A typical output of the failure was like :

+ chroot /var/tmp/CI/all-debian/install/D7-H.1.1.0/openstack-full dkms build -m openvswitch -v 2.0.1 -k 3.12-0.bpo.1-amd64

Kernel preparation unnecessary for this kernel.  Skipping...

Building module:
cleaning build area....(bad exit status: 2)
./configure --with-linux='/lib/modules/3.12-0.bpo.1-amd64/build' && make -C datapath/linux.......(bad exit status: 1)

Error! Bad return status for module build on kernel: 3.12-0.bpo.1-amd64 (x86_64)
Consult /var/lib/dkms/openvswitch/2.0.1/build/make.log for more information.
openstack-full: Fixing DKMS compilation issue

edeploy's makefile does export ARCH= set to amd64.

When dkms is called, the ARCH variable is taken in account by the
kernel modules compilation process and try to find a arch/$ARCH inside
the kernel tree.

arch/amd64 doesn't exist inside a kernel tree so the build process
fails.

This patch unset the ARCH variable before the dkms call to let the
kernel detect the proper one aka x86.

This solve the build process issue on dkms.