(FACT-697) Add confine to dhcp_servers for nmcli

This patch adds a confine on dhcp_servers to ensure that, not only is
nmcli present, but actually is connected to NetworkManager.

This fixes the issue where facter is throwing an error of
'NetworkManager is not running' at each run of facter.

Closes: FACT-697
Revert "(FACT-658) facter doesn't parse gnu uptime output"

This reverts commit b8ea20db6826c906f9c527a7dbf9aa04fbfae960.

After some discussion with Josh Cooper, we agreed that relying
on the exact path to uptime for all platforms is the wrong approach,
so we're reverting.

Two possibilities are:
1) Set the exact path to uptime for Solaris only
2) Make the uptime parsing resilient to gnu uptime output
(maint) doc fix - there is no Facter::Util::Execution

Yard docs had a reference to `Facter::Util::Execution` (does not exist) instead of `Facter::Core::Execution`
Merge pull request #817 from jhoblitt/maint/doc_typo_facter_util_execution

(maint) doc fix - there is no Facter::Util::Execution
Merge pull request #811 from onyxpoint/FACT-697

(FACT-697) Add confine to dhcp_servers for nmcli
(FACT-697) Fix nmcli command line usage for versions < 0.9.9.

The code to confine the dhcp_servers fact to when NetworkManager's
status is not "unknown" used a command line syntax that is only
available in nmcli versions >= 0.9.9.

This commit uses the older command line syntax when the nmcli version is
less than 0.9.9.
Merge pull request #818 from peterhuene/fact/697

(FACT-697) Fix nmcli command line usage for versions < 0.9.9.