Fix usage information for -c to indicate a conf.d directory can be used

Since puppetdb can accept a directory of configuration, the current usage
information for the -c switch is not completely accurate. This patch
changes the usage information to indicate a directory can be used for
both the 'services' and 'benchmark' sub-commands.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #564 from kbarber/config_switch_takes_directory

Fix usage information for -c to indicate a conf.d directory can be used