logutils: handle yum logginglevels correctly

This lets us get crazy amounts of logging out of yum, if needed.
improved log messages during depsolving

Set up a verbose_logger so we can get log messages out of yum,
improve/clarify log messages from pkgAdded, and fix a backward log
message in the depsolve callback. (fixes #19)
fix instrepo key import

We need to import instrepo's key(s) *after* we create gpgdir, obviously.
Move it into check_signed_file() so it happens at the right time.
refactor: drop _import_key()

This only got called from one place, and we can do a better job than
this. Move the code into the place it was called from.
refactor: add init_keyring, list_keyring, import_key

More code refactoring. No behavior changes except perhaps slightly
different log messages.
refactor: add _setup_keyring()

Moving this into its own function makes logical sense - it's possible we
might want to use it elsewhere, but it's also much easier to understand
what's actually going on here.
fix rm_rf() for non-directories

C'mon, let's use the BIG hammer!!
use a clean gpg keyring every time

This makes sure we don't end up with stale/no-longer-trustworthy keys
stuck in a cached GPG keyring.
download.py: improve logging from check_keyfile etc.

Makes the logs clearer and easier to read.
check_keyfile: fix false positive for RPM trust

It turns out that keyInstalled returns -1 if the keyid isn't found in
the RPM database... which evaluates to True. Yikes.

This patch makes check_keyfile properly reject keys that came from
packages signed with an *untrusted* key.
Clarify log messages for cleanly-handled exceptions

Just 'cuz there's a traceback in the log doesn't mean we crashed.
Make it clear whether or not this is a crash or a simple failure.
Add SystemUpgrade.asciidoc

This is a draft of a distro-neutral spec for doing System Upgrades with
systemd. fedup and fedup-dracut implement this behavior but eventually
the parts described here should be upstream.

'til then, the docs should go somewhere, right?
Merge branch 'wgwoods-fedup-master' into dashea-fedup-master
Merge the latest code from master into the retro branch
Merge the current fedup retro code into rhelup