Make Ubuntu/Debian depend on python-selinux (Closed GH:#93)

Signed-off-by: Adam Stokes <hackr@cypherbook.com>
Merge pull request #94 from battlemidget/feature

Make Ubuntu/Debian depend on python-selinux (Closed GH:#93)
Rename command output collection interfaces

The method names for collecting external output are ugly and
awkward to type. Rename them to be more consistent with the file
collection API and easier to remember and type.

This change touches virtually every plugin class but is a very
simple search/replace (plugin changes were done with sed).

Fixes Issue #92.
Separate archive classes from utilities module

This is the start of the archive reworking to address Issue #86.

In this commit the Archive class hierarchy is separated from the
generic code in the utilities module and moved into its own
'archive' module. This is taken from the changes committed to
Jesse's directory_archive branch.

The next step is to decide how we will handle the problems
discovered with the in-line tarfile generation (and similar
problems affecting Zip files) and whether to revert to building
an in-filesystem tree (as Jesse's current branch does) or whether
to fix the problems with in-lining post-processing hooks.
Fix import statement in archives_test.py
Disable archive recursion tests for TarFileArchive

The TarFileArchive class currently doesn't allow recursive
directory addition using the add_file() interface - disable those
tests for now.
Force LC_ALL=C for external commands

Commands run by sos inherit the environment of the user running the
program. This includes locale settings meaning that collected
output is subject to local language, sorting and formatting
customisations that can be undesirable when post-processing or
parsing of the gathered data is required.

Force all external processes to use the 'C' locale.
Make add_file() more robust

The TarFileArchive with SELinux does not pass the full test
archive_tests suite. Catch an uncaught exception when stat'ing a
non-existent path and do not allow empty paths in add_parent().