Allow custom sync config locations

[garfieldius]

Add a check for the environment variable "SYNC_CONFIG" which is used to find a configuration file. This allows to place the files at a different location than the acutal project, making re-use of configurations easier.

Example

SYNC_CONFIG="$HOME/sync-projecta.yml" ct sync:server staging

The default behaviour is not altered by this change.

[josefglatz]

Would be nice to get such a feature into clitools 👍

[mblaschke]

Why not as option? ct sync:server staging --config=sync.yml