(FACT-766) Add weight param to DirectoryLoader#new

Allows users of this class to specify a weight for the created facts,
without having to hack around the use of a constant.
(FACT-766) Fix DirectoryLoader default weight spec

This test never worked properly; inverting the `-` to a `+`, giving the
internally-loaded fact a weight *higher* than the external fact didn't
break the spec. Changing this to use `collection.add` rather than the
global `Facter.add` fixes the issue.
(maint) Replace deprecated .exists? with .exist?

`File.exists?` is deprecated and should be upgraded to the correct format
(maint) More clearly nest local variables

`nic` was used for both the outer and inner local variable, which is confusing in terms of which scopes belong when creating the `interface_names`, so this PR is designed to clarify that code and remove any ambiguity
Merge pull request #824 from dgoodlad/directory-loader-configurable-weight

(FACT-766) Directory loader configurable weight
* (FACT-766) Add weight param to DirectoryLoader#new
* (FACT-766) Fix DirectoryLoader default weight spec
(FACT-715) Fix LXC detection code on old OpenVZ

LXC detection code raises errors on old OpenVZ. Avoid terminating by
catching failures and returning false.
Merge pull request #826 from MikaelSmith/maint/master/remove-deprecated-exists-method

(maint) Replace deprecated .exists? with .exist?
Merge pull request #827 from MikaelSmith/maint/master/code-disambiguation

(maint) More clearly nest local variables
Merge pull request #828 from MikaelSmith/bug/master/FACT-715-fix-LXC-detect

(FACT-715) Fix LXC detection code on old OpenVZ
(FACT-770) Added default interface support for AIX
(FACT-770) AIX >= 4.3 Networking improvements
Merge pull request #829 from MikaelSmith/imp/master/FACT-770-improve-network-fact-AIX

(FACT-770) Networking improvements for AIX