Allow for using custom s3 host with lfs server

Allow for using custom s3 host with lfs server

By default, it will generate hostname using the aws region passed to the
constructor.

This will allow for easier testing, since you can just spin up a local
minio (or other s3-compatible storage service) instance and point the
application at that for the storage mechanism.

It will also allow for storing lfs objects on-prem.

Change-Id: I2566b1fcce58f3d306ddd23a8da702ef5a451c7b
Signed-off-by: Pat Long <pllong@arista.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>