build/base.install: support grub2 under RHEL7
init.common:
 - launch sshd under RHEL7
 - allow PCI ids to be in upper case
 - support virtio modules when they are not loaded automatically.
build/init:
 - create /etc/default/grub if it doesn't exist
 - create /boot/grub2/grub.cfg if in grub2 2 prefix mode
ansible/edeploy-install.yml: fix detection of RHEL
build/deploy.install: fix pxe configuration under RHEL7
build/repositories: do not add rhel-x86_64-server-optional-7 in add_rhn_repository