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