Add multi-tenant support

[wansors]

Instead of defining a list of git servers (each for a different tenant), It will be nice to have support to set the parameter {tenant} on the config file. For example:
uri: https://github.com/wansors/config-{tenant}.git

The {tenant} variable could be obtained from the request in the following format: tenant(_)application.

Then a instante of the config server can serve multiple tenants without further modifications on the config file.