add --releasever=/ to mkimage-yum.sh

I didn't realize the commit required a Docker-DCO so it failed the travis-ci build. So I removed the commit from my forked repo. Now it looks like there is a pull request with no commit. So here it is again:

Needed to add '--releasever=/' flag to run yum groupinstall on Centos7 (didn't try on anything else). This snippet from yum man page explains why:

```
Note: You may also want to use the option --releasever=/ when creating the installroot as otherwise the $releasever value is taken from the rpmdb within the installroot (and thus.  will  be  empty,  before  creation).
```

Docker-DCO-1.1-Signed-off-by: Matt Schurenko <matt.schurenko@gmail.com> (github: mschurenko)

contrib/mkimage-yum.sh

@@ -57,7 +57,7 @@ mknod -m 666 "$target"/dev/tty0 c 4 0
 mknod -m 666 "$target"/dev/urandom c 1 9
 mknod -m 666 "$target"/dev/zero c 1 5
 
-yum -c "$yum_config" --installroot="$target" --setopt=tsflags=nodocs \
+yum -c "$yum_config" --installroot="$target" --releasever=/ --setopt=tsflags=nodocs \
     --setopt=group_package_types=mandatory -y groupinstall Core
 yum -c "$yum_config" --installroot="$target" -y clean all