Add support for wordpress network-mode

[graphiclunarkid]

Wordpress network-mode allows multiple separate blogs to be served from
the same instance of the software. These can be served as separate
subdirectories or separate subdomains. With the latter option, a plugin
is included to allow separate apex domains to be mapped to each
subdomain.

Wordpress.com is the largest example of wordpress running in network
mode.

This branch also:

• Makes configurable the domain and subdirectory from which wordpress is served
• Refactors the wordpress role
• Includes defaults for all optional parameters
• Includes handlers to ensure these are run when the role is executed in
  isolation
• Updates the role documentation to explain the new options
• Updates wordpress to v4.4.2

Resolves #66

[kheops2713]

• nginx vhost file is redundant with nignx role vhost file ("websites" variable)
• other actions are redundant with "websites" variable, like creating domain directories
• add meta/main.yml file to autoinclude nginx and php-fpm role
• try re-adding "alias" nginx directive for wordpress subdomain configuration where location is '/'
• make sure PHP is called for all domains serving a wordpress site, in case of a multisite with several domain names, by copying the nginx included configuration location / into every domain's configuration path
• remove handlers redundant with nginx role