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

openstack/sahara-image-elements,uladz/sahara-image-elements,openstack/sahara-image-elements,uladz/sahara-image-elements