Support php_admin_(flag|value)s

Add support for php_admin_flags and php_admin_values to both
<VirtualHost> and directory context. Basic documentation & tests
for both cases included.

This Fixes #463
Fix $ports_file reference in Namevirtualhost.

This define was pointing at the params::ports_file version of the
variable, rather than the one defined in the main class, making it
impossible to correctly override.
Fix $ports_file reference in apache::listen.

This was pointing at ::params::ports_file instead of the main class,
causing relocation of the ports file to fail.
Merge pull request #532 from apenney/quick-fix

Fix $ports_file reference in Namevirtualhost.
Merge pull request #481 from igalic/php_admin

Support php_admin_(flag|value)s
Support environment variable control for CustomLog