Add optional {{ source }} to config files

[0xf10e]

As proposed in the "{{ source }} in formulas" thread on [salt-users] with the __formulas pillar-key suggested by @whiteinge.

By default (if pillar[__formulas:print_template_url] and pillar[openssh-formula:print_template_url] are left unset) a block like the following will be added to the config files:

# Template used to generate this file:         
# salt://openssh/files/ssh_config

The preference can be set pillar[__formulas:print_template_url] for cross-formula purposes with pillar[openssh-formula:print_template_url] as a local override.