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

* correctly set dependencies in configuration
* update RPM repo key
* add tests for passenger installation as nginx module
* update test matrix

(...) for snippets filename.
BREAKING CHANGE: Users have to modify their pillar
according to this commit. Users MUST append '.conf' for their
existing managed snippets.

First implementation of TOFS for:
nginx_config, passenger_config, servers_config and snippets
Introduced tplroot on modified files as well
Fixed GH link of nginx.conf file in docs/TOFS_pattern.rst
Fixed test for snippets name pillar

BREAKING CHANGE: all previous `nginx` based configurations must be reviewed;
`nginx.ng` usage must be promoted to `nginx` and any uses of the original
`nginx` will have to be converted.