Allow a server not to have Let's encrypt domains in pillar

Summary:
The salt['pillar.get'] method allows to specify a default value.
That allows to avoid to require to add empty lists in pillar when
no certificate domains have been specified.
Don't require in the pillar to define certificates_letsencrypt

Test Plan: Tested on Ysul, works fine, for loop is well ignored.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1115