Add documentation for new parameters.
Convert some classes to not be parameterized.

We're following (where possible) the pattern of having parameters only
exist on interface classes.  As ntp::config and ntp::install are just
internal, private, classes, these don't make sense to allow changes to.
Rewrite the spec testing.

A lot of changes here, primarily to unify the spec testing in a single
spec file to reflect the changes to the private ntp::config and
::install classes.  As a side effect make sure we properly test each
supported distribution (including Gentoo properly) and prefer to use
'content' => /blah/ rather than other methods of testing file contents.