chan_dahdi: enable ignore_failed_channels by default

If ignore_failed_channels is set to "true" for a channel, the channel
will continue to be configured even if configuring it has failed.

This allows Asterisk to start before all the DAHDI initialization is
done and thus not force the starting order dahdi -> asterisk.

Review: https://reviewboard.asterisk.org/r/3063/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@404542 f38db490-d61c-443f-a65b-d21fe96a405b