Add Validation and rspec tests for $error_log

Without this commit, there is no validation or testing for the
$error_log parameter in the apache::vhost type.

This commit add boolean validation to the apache::vhost type. It also
adds two rspec tests to test the proper function of the parameter.
Merge pull request #186 from trlinkin/refactor_vhost_logformat

Add Validation and rspec tests for $error_log