Add element to rebuild dracut enabling modules

To support TripleO whole disk images (with enabled
security), we need lvm and crypt modules.
So creating that element in tripleo, that will install the
needed packages, and will rebuild the image to add the
modules we need.

Change-Id: I1b0dd384a1981acffd64652708ec21323f5e795a
Partial-Bug: #1655681

elements/overcloud-dracut/README.rst

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+================
+overcloud-dracut
+================
+Rebuilds initramfs adding all the modules needed to support
+TripleO needs in terms of booting whole disk images with
+enabled security. We need lvm and encryption on the images,
+so adding related packages and modules to enable those.

elements/overcloud-dracut/element-deps

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+package-installs
+select-boot-kernel-initrd

elements/overcloud-dracut/finalise.d/98-regenerate-initramfs

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+#!/bin/bash
+
+if [ ${DIB_DEBUG_TRACE:-0} -gt 0 ]; then
+    set -x
+fi
+set -eu
+set -o pipefail
+
+RET=$(select-boot-kernel-initrd)
+KERNEL_VERSION=`echo ${RET%:*} | sed 's/vmlinuz-//g'`
+RAMDISK=/boot/${RET#*:}
+
+dracut --force --add "crypt lvm" $RAMDISK $KERNEL_VERSION

elements/overcloud-dracut/package-installs.yaml

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+cryptsetup:
+lvm2: