(TK-131) Remove precondition from `configure-logger!`
(TK-131) Remove precondition from `configure-logger!`

Previously, the `configure-logger!` function had a precondition asserting that
the `logging-conf` argument passed to it matched one of five different types.
The argument is passed through to the `doConfigure` method for the
JoranConfigurator class.

The benefit to having the precondition is a more explicit error message in the
case where `logging-conf` is of a type that `doConfigure` doesn't accept.
However, the downside to this is that we have to have a long list of all the
possibilities, which we previously got wrong, thereby artificially
constraining what could be used to configure logging. Thus, this commit
removes the precondition completely and updates the relevant docstring.

With this change, the error message if an argument of incorrect
type is used is not very informative:

    java.lang.IllegalArgumentException: No matching method found: doConfigure
    for class ch.qos.logback.classic.joran.JoranConfigurator

However, in order for someone to encounter this, they would have to be
configuring logging from code, not from a config file, and could reasonably be
expected to already be looking at the documentation for JoranConfigurator, or
to be able to familiarize themselves with this easily.

camlow325/trapperkeeper,scotje/trapperkeeper,puppetlabs/trapperkeeper,rbramwell/trapperkeeper