ncm-spma: support for dnf backend schema to support main_options

[aka7]

Support for spma/main_options in dnf backend, used by aii to configure tmp yum.conf during ks post.

So that we can set:

"/software/components/spma/main_options/best" = 1;

• Why the change is necessary. to be able to set best=1 in aii yum.conf.
• What backwards incompatibility it may introduce. None. just schema update.

Resolves #1702.