Add first tests for live configuration reload mechanism

These commit introduces tests that cover basic cases of configuration
reload related to changing XMPP domain.

Currently all the work on configuration reload is kept in
https://github.com/esl/MongooseIM/tree/conf-reload
Use nullary reload_local command

(cherry picked from commit 2c2671c823afe39ad4ec8ab17ac6c32307702d3a)
Make the tests run independently

(cherry picked from commit 1af297cd47af78a9efe151ac191638c7d496ecff)
Print and log external command errors

(cherry picked from commit c379b166ded1d8b8a3161a816479913ee2ac3b4a)
Cleanup spurious whitespace

(cherry picked from commit f16e63e06589c65d6a1bd4be5150df8b662f9050)
Include conf_reload suite in specs
Merge pull request #86 from esl/conf-reload

Tests for configuration reload