gc: Add --quiet flag

[thusoy]

Environment

Linux 4.9.0-6-amd64 x86_64
--
PRETTY_NAME="Debian GNU/Linux 9 (stretch)"
NAME="Debian GNU/Linux"
VERSION_ID="9"
VERSION="9 (stretch)"
ID=debian
HOME_URL="https://www.debian.org/"
SUPPORT_URL="https://www.debian.org/support"
BUG_REPORT_URL="https://bugs.debian.org/"
--
systemd 232
+PAM +AUDIT +SELINUX +IMA +APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ +LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD +IDN

What did you do?
rkt gc in a cron job

What did you expect to see?
No output for successful execution.

What did you see instead?
Lots of informational output like Garbage collecting pod and pod %q not removed: still within grace period. Output is written to a mix of stdout and stderr, and when run from a cron job any output will cause cron to send a email report.

Either all output should be reviewed so that all status messages go to stdout and can be silenced in the crontab, or a flag like --quiet should be implemented to suppress all informational output.