(pup-1766) Add support for multiple defaultfor's

Prior to this change, it was possible to specify a defaultfor to
cover, say, multiple osfamily's e.g. with:
    defaultfor :osfamily => [:os1, :os2]
Or it was possible to specify a more complex hash, e.g. with
    defaultfor :osfamily => :os1, :operatingsystemmajorrelease => "42"

But if more specificity for just one of the operatingsystem values was
needed, defaultfor couldn't support that. This change allows for this, e.g.:
    defaultfor :osfamily => :os1  # all variants of os1
    defaultfor :osfamily => :os2, :operatingsystemmajrelease => "42"  # only release #42 of this os

To support this change Provider defaults is now an array of hashes (formerly,
it was a merge of the hashes provided by all defaultfor invocations), which requires
commensurate updates to the specificity method and the dochook.

There are also some new spec tests for representative defaultfor combinations, and
two spec tests needed to be updated with the expectation that Facter will be invoked
to assess defaultprovider specificity.
(pup-1766) Set systemd as a default provider for the service type on rhel7