dnf-context: Do not modify global configuration when setting options

[jan-kolarik]

During the work on the #1635, I discovered several places where option setters in dnf-context are modifying the global configuration and therefore affect other context instances:

dnf_context_set_best();
dnf_context_set_install_weak_deps();
dnf_context_set_allow_vendor_change();
dnf_context_set_keep_cache();
dnf_context_set_zchunk();
dnf_context_set_plugins_all_disabled();

Maybe there are other places, I didn't investigate deeply.

[jan-kolarik]

It seems it might be on purpose looking at the #998.