(#3757) separate settings for agent lockfile paths

There are now two different lockfiles for agents; one
for indicating that an agent is administratively
disabled, and one that indicates that an agent run
is already in progress.

In previous revisions, the paths for these lockfiles
was being built from a single puppet setting via
naming convention. This commit does the following:

1. Creates two settings (:agent_running_lockfile and

:agent_disabled_lockfile) instead of the previously
existing one (:puppetdlock).

2. Removes the Configurer class from the chain of

command when the Agent class is determining the
lockfile path.