Add systemd time validation to the chef_client_systemd_timer resource

[tas50]

We really want to make sure that folks input valid time values for these timers. If they put in bad data our systemd_unit validation will actually pass. It turns out that systemd-analyze verify which we run will throw error warnings to the shell, but doesn't fail if the time values are bad:

root@timnas2:/home/tsmith# systemd-analyze verify /etc/systemd/system/chef-client.timer
[/etc/systemd/system/chef-client.timer:8] Failed to parse timer value, ignoring: 1mi
root@timnas2:/home/tsmith# echo $?
0