diskimage-create.sh: try harder with provides in rpm check

When checking whether a package is installed using rpm, try harder using
the wanted name as provides of a real package.  This fixes the check for
installed python-argparse on CentOS 7, as it is provided by the main
python package.

Thanks to: Brandon James.
Closes-Bug: #1494790

Change-Id: I88d1d0293e3dde9c6087e6e0209f234e922f52ce

diskimage-create/diskimage-create.sh

@@ -301,7 +301,9 @@ is_installed() {
         dpkg -s "$1" &> /dev/null
     else
         # centos, fedora, opensuse, or rhel
-        rpm -q "$1" &> /dev/null
+        if ! rpm -q "$1" &> /dev/null; then
+            rpm -q "$(rpm -q --whatprovides "$1")"
+        fi
     fi
 }