Add param for separate RootCA

Closed username-removed-20268 requested to merge iavael/gitlab-ci-multi-runner:master into master

Added -x/--rootca param to define non-system rootca store. It may be needed, fo example if we have internal ca, which we don't want to install systemwide for security reasons or, in my case, if we have selfsigned certificate on gitlab-ci server (we pass this certificate as rootca store and so workaround bugs like #150 (closed) and #71 (closed))

Since UnregisterRunner is totally broken (#39 (closed), it ignores cli options and tries to read config before loading it, so URL and Token params are always empty), I didn't touch it, because fixing it is unrelated to this MR and there are several ways to do this.