Fixes regression introduced by recent change to avoid double call to flag.Parse()

[pstibrany]

This PR fixes regression introduced by #1997.

When using flag.ContinueOnError, Parse() method on *flag.FlagSet returns errors, instead of panicking or exiting the program.

My assumption was that it actually continues, but that's not the case. When cortex was called with ./cortex -target=distributor -server.http-listen-port=8001 -server.grpc-listen-port=9001 -config.file=./config.yaml, Parse() method returned error on first unknown parameter (which is -target=... here, as we only look for -config.file at first), and then config file parameter was not found.

To actually find the -config.file parameter, we now call flag.Parse repeatedly with reduced set of arguments, until configFile is set to non-zero string or we run out of arguments.

[jtlisi]

LGTM

[pracucci]

LGTM. I tested with -config.file at the beginning and end of arguments, and now looks working as expected. Thanks for the quick fix.