Allow using $http_cfg_append with list of lists

It should be possible to include array of arrays in order to have
duplicate configuration keys (when permitted), like:

    nginx::http_cfg_append:
        - [include, /etc/nginx/vhosts/*.ngx]
        - [include, /etc/nginx/vhosts/*/*.ngx]
Merge pull request #369 from Motiejus/validate_hash

Allow using $http_cfg_append with list of lists