Handle non-script grub2-install

grub2-install has been re-written [1] so isn't always a shell script,
so just exec it directly.  Also use $() as that follows the rest of
the file.

[1] http://git.savannah.gnu.org/cgit/grub.git/commit?id=cd46aa6cefabd7b0fcb15cd614577e1ab8c7a841

Change-Id: Ib4fd7c6854fb485c0da6a79babf4a3d45a8a0f2c
Initial centos7 support

Initial support for a centos7 image.

This is separate to rhel7 because the major differences are things
like repo and image locations, which are always going to be different.
We should merge any real changes into the redhat-common layers.

Apart from the added support files in centos7/*, the other change is
mostly modifications to redhat-common's extract-image to handle
different partition layouts of the centos7 image.

Change-Id: I943abe5ff0a803f36eda266a79af0d9220edcae7
Update RHEL7 element since release

This patch updates the RHEL7 element to work
properly now that it has changed from beta to
release status.  It includes a repo update to
locate os-config rpms not in epel yet and an
update to the latest cloud image name.

Change-Id: I05607d62193c8f8fb1c1bece9872d456b19c5a08
Revert "Cleanup apt cache after grub install"

This calls chroot as a non-root user, which will always fail with
EPERM.

This reverts commit ab2d1a31f2177b26aada075201712b0061a1b771.

Change-Id: Ie674fef694ad66e1ebc22083dec2a0bc34371e7b
Merge "Revert "Cleanup apt cache after grub install""
Merge "Update RHEL7 element since release"
Merge "Handle non-script grub2-install"
Merge "Initial centos7 support"