adds a way to specify the configstore to use

[nicoovh]

Is your feature request related to a problem? Please describe.
I am deploying utask in an environnement that has already defined CONFIGURATION_FROM venv value to a location with limited rights.
Since utask is using configstore.DefaultStore that basically use this venv to find its configuration I can't configure it properly.

Describe the solution you'd like
A better approach would be to look for a UTASK_CONFIGURATION_FROM venv, and if defined, initialise the configstore.Store on that venv.
If not defined, use the configstore.DefaultStore like before.

Describe alternatives you've considered
The only workaround I have is to fill the defined location with utask configuration values, but some configuration could collide at some point, (especially those without UTASK_ prefix)

Additional context
requires an evolution of configstore: ovh/configstore#32