dp: set be_ctx->provider as part of dp_init request
    
    Backend context is overused inside sssd code even during its initialization.
    Some parts of initialization code requires access to be_ctx->provider so we
    must make it available as soon as possible.
    
    Better solution would be to always use 'provider' directly in initialization
    but this makes it safer for any future changes as one does not have to keep
    in mind when it is safe to use be_ctx->provider and when not. Now it is
    always safe.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3812
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>