FR: support --no-email option for ltm commands

[lrumancik]

When running LTM commands for selftests, we likely don't want to receive emails. Right now, --no-email is only supported for pure gce-xfstests commands (gce-xfstests -c ...) but not for ltm commands (gce-xfstests ltm -c ...). If you attempt to use --no-email for an ltm command, the tests will run but sharder.emailReport() will panic due to no email address causing the results not to get packed and uploaded to gce.

sharder.go:

func (sharder *ShardScheduler) finish()  {
    sharder.log.Debug("Finishing sharder")

    sharder.aggResults()
    sharder.createInfo()
    sharder.createRunStats()
    sharder.genResultsSummary()

    if !sharder.reportKCS {
        sharder.emailReport() -> panic'ing because no where to send email
    }

    sharder.packResults()
}