Handle empty package list for install_packages

Currently if the install_packages macro is run with an empty
package list, it will add a yum or apt-get command with no
packages listed.

This bug fix aims to omit this line when no packages have
been given, or, the operator wants to use the "_override" /
"_remove" functionality to disable all packages being
installed in a Dockerfile.

Co-Authored-By: Paul Bourke <paul.bourke@oracle.com>
Change-Id: Ifaaaebfccc3adb0f2f68a35ac08e59378bc87fdb
Closes-bug: 1612446