fix(snippets): ignore servers or snippets when undefined

nginx.servers_config wants a lightened copy of the nginx map
to render as json; but, when it was trying to remove the
servers and snippets keys from the map it assumed their presence,
causing a KeyError if they were not present by its use of .pop().

While wrapping these in an "if" clause would likely be more
correct, along with replacing .pop() with del (if jinja even
supports that) the simplest change here is to just specify a
default value for .pop(), which obviates the KeyError.

Fixes #274

nginx/servers_config.sls

 {% set server_states = [] %}
 {#- _nginx is a lightened copy of nginx map intended to passed in templates #}
 {%- set _nginx = nginx.copy() %}
-{%- do _nginx.pop('snippets') %}
-{%- do _nginx.pop('servers') %}
+{%- do _nginx.pop('snippets') if nginx.snippets is defined %}
+{%- do _nginx.pop('servers') if nginx.servers is defined %}
 
 # Simple path concatenation.
 # Needs work to make this function on windows.

nginx/snippets.sls

 
 {#- _nginx is a lightened copy of nginx map intended to passed in templates #}
 {%- set _nginx = nginx.copy() %}
-{%- do _nginx.pop('snippets') %}
-{%- do _nginx.pop('servers') %}
+{%- do _nginx.pop('snippets') if nginx.snippets is defined %}
+{%- do _nginx.pop('servers') if nginx.servers is defined %}
 
 nginx_snippets_dir:
   file.directory: