(FACT-598) Use `hostname -f` for `fqdn` if available.

(This is a retry of pull request #680. Changed branch. Added JIRA ticket reference.)

Some systems have a hostname other than the concatenation of `hostname`
and `dnsdomainname`. Or put differently, the `hostname -s` is not equal
to `hostname`:

    $ hostname -f
    systemX.locationY.domain.tld
    $ hostname
    domain-systemX

This keeps the local hostname short yet detailed enough.

In that case, the right FQDN would be obtained by concatenating the
(short!) `hostname -s` with `dnsdomanname`:

    $ hostname -s
    systemX
    $ dnsdomainname
    locationY.domain.tld

However, `facter fqdn` returns this:

    $ facter fqdn
    domain-systemX.locationY.domain.tld
    ^-- wrong, should be: systemX.locationY.domain.tld

This patch changes things so that on systems with `hostname -f`, that
value is used instead.