SignalFx requires session token when fetching API Keys

[csolidum]

Last year, SignalFx changed their api so that org tokens can no longer be used to fetch other org tokens as mentioned in their docs. This breaks token fetching whenSignalfxDynamicPerTagAPIKeysEnable is enabled in the SignalFx sync, since it relied on the SignalfxAPIKey, which is normally an org token, to fetch these keys. In order to fix this, veneur would need to generate a session token using a valid email/password.