install Yast packages via dependencies

- there can be different packages in SLES and openSUSE inst-sys
- make some branding packages optional (SLE branding has a different
  package set than openSUSE)

This change also needs changes in the `spec` file, see [diff in Devel:YaST:Head](https://build.suse.de/package/rdiff/Devel:YaST:Head/installation-images?opackage=installation-images&oproject=SUSE%3ASLE-12%3AGA&rev=9) IBS. (BTW why is the spec file not in Git? I could include the change here as well.)

This also fixes P1 [bnc#861074](https://bugzilla.novell.com/show_bug.cgi?id=861074) - please merge and submit to SLE12 **ASAP** (for Alpha3), thank you a lot!