Handle /v1 suffix in SUMOLOGIC_BASE_URL

[pmalek-sumo]

When user uses base URL like https://api.sumologic.com/api/v1/ instead of https://api.sumologic.com/api/ then provider instead of handling that prints the following

Error: {"id":"O6SOJ-9BDH3-YQG04","errors":[{"code":"unauthorized","message":"Credential could not be verified."}]}

It would be better to warn the user that the provided API URL doesn't seem right.

[mccartney]

... or correct it automatically behind the scenes.