WIP Default configuration and external config file

This is still work in progress. Load a default configuration with sane
defaults. Can specifiy a configuration file overriding these values
by setting the environment var PONYEXPRESS_CFG to a filepath.

Signed-off-by: Christoph Kassen <c.kassen@telekom.de>
Default database matches packaging installation db path

The deb package creates the path /var/lib/ponyexpress. Put the db
into that path by default.

Signed-off-by: Christoph Kassen <c.kassen@telekom.de>
Fix env var loading

Flask is taking care of loading the config specified in the given env var.
No need to translate this into a file path ourselves.

Fixes the Travis build.

Signed-off-by: Christoph Kassen <c.kassen@telekom.de>
Configuration import not required

Signed-off-by: Christoph Kassen <c.kassen@telekom.de>
Merge pull request #18 from TelekomCloud/feature/external-config

Feature/external config