Store.InitFromEnvironment shall allow to specify another ENV VAR name

[nicoovh]

Is your feature request related to a problem? Please describe.
When two tools are using configstore in a same ENV, their configuration may collide because the ENV var name is global.

Describe the solution you'd like
The Store.InitFromEnvironnement shall have the VENV name as parameter, default to CONFIGURATION_FROM

Describe alternatives you've considered
None

Additional context
code location:

configstore/store.go

Line 48 in 3cb5c15

func (s *Store) InitFromEnvironment() {

related issue on utask: ovh/utask#361