Rather than returning an error that requires pattern-matching from
`parseChallenge` when the challenge header requires basic
authentication, return a distinguished error value. This makes checking
for this error a bit easier.
This commit also updates the check in `r.Headers` to use the new error
value and adds a couple of regression tests.
* Add flag to trust ssl certificates signed by unknown authority
* Use registry http client instead of pure http client
* Add Bearer token only if required
* Create clair client instance with configurable debug option
* Limit number of parallel vuln scann´s to 20 to reduce load
* No need to throttle anymore because parallism is limited
* Make number of workers configurable
* During first run do not create clair vulns report details