yum-minimal: Disable excludes when installing pkg manager

Because we are using the building platform's "yum" to do the initial
install into the chroot, it is affected by the base-system's
/etc/yum.conf.

pip-and-virtaulenv in I82acb865378a0fa5903a6267bfcee0e2962eced0 added
"exclude=python-pip..." in /etc/yum.conf to stop the package manager
overwriting the installed pip.  Now our CI images have built with
this, we are now picking up this exclude on centos.  Since on F24
dnf->python->python-pip we end up failing to build the the chroot
because python-pip can not be satisifed.  In a general sense, however,
this could be caused by any configuration put into /etc/yum.conf that
is incompatible with installing into the chroot.

yum has the option to disable all excludes which is used here.  This
seems to be the best way to isolate the chroot install from any
excludes that may have been done on the base system for various
reasons.  I did consider using a completely separate yum.conf we ship
with dib ... but let's start simple.

This should fix the current gate failures on centos

Change-Id: I4e4cc8ed09a29c4057ade34ea93025139e191bf5

elements/yum-minimal/root.d/08-yum-chroot

@@ -156,6 +156,7 @@ function _install_pkg_manager {
         fi
 
         sudo -E yum -y \
+            --disableexcludes=all \
             --setopt=cachedir=$YUM_CACHE/$ARCH/$DIB_RELEASE \
             --setopt=reposdir=$TARGET_ROOT/etc/yum.repos.d \
             --releasever=$DIB_RELEASE \