add gr_apache_conf_template to graphite class

Allow the Apache vhost config template to be specified, making it easier
for users to deploy custom templates. Defaults to previous hard-coded value
graphite/etc/apache2/sites-available/graphite.conf.erb
added docs to README for gr_apache_conf_template
Merge pull request #84 from genome-vendor/apache_template

add gr_apache_conf_template