to reduce duplication of constants

... by providing a pillar string. I developed this for use in
combination with ext_pillar and file_tree to deploy letsencrypt
certificates.

default value if dhparam isn't defined in the pillar.

Improvement - Disable reload of nginx when enabled=False (previously the nginx was always reloaded if settings.config != None; even with enabled=False)

In order to improve security and ease of use, added creation/generation
of dhparam file.

Removed comment after testing that this change works with xenial

Fixes #70