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
Fix mod::prefork dependencies

A dependency on the package resolves the following error:
"Error: /Stage[main]/Apache::Mod::Prefork/File_line[/etc/sysconfig/httpd
prefork enable]: Could not evaluate: No such file or directory -
/etc/sysconfig/httpd"
Move mpm_worker 'ServerLimit' line in ERB

It turns out that the order of MPM directives is important in certain
cases. Without this commit, the 'ServerLimit' directive was being
processed too late to be used in calculations for 'ThreadsPerChild'.

This commit moves the 'ServerLimit' directive to the top of the
directives in the ERB. This was based on Apache documentation and an
example config of theirs.
Merge pull request #191 from trlinkin/master

Move 'ServerLimit' line in worker.erb
Use correct template for dav_fs module
Merge pull request #188 from nanliu/prefork

Fix mod::prefork dependencies
Merge pull request #194 from mxey/master

Use correct template for dav_fs module
Add proxy_pass parameter
Merge pull request #193 from oxilion/proxy_pass

Add proxy_pass parameter