Suppress misleading warn. in openbsd provider

I have observed that `pkginfo` command used as `listcmd` by `ports`
package provider sometimes outputs extra messages to stdout (the message
starts with `Updating the pkgdb`). This causes puppet to issue its own
warning as the line obtained from `listcmd` is not a valid package name.

This patch suppresses this spurious message.
Merge pull request #2051 from ptomulik/fix/package_openbsd_spurious_warning

Suppress misleading warn. in openbsd provider