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
Merge "Handle non-script grub2-install"
Merge "Initial centos7 support"