cert: allow cert expiry to be set in config

sample-lnd.conf

@@ -52,6 +52,11 @@
 ; change.
 ; tlsautorefresh=true
 
+; The duration from generating the self signed certificate to the certificate
+; expiry date. Valid time units are {s, m, h}.
+; The below value is about 14 months (14 * 30 * 24 = 10080)
+; tlscertduration=10080h
+
 ; Do not include the interface IPs or the system hostname in TLS certificate,
 ; use first --tlsextradomain as Common Name instead, if set.
 ; tlsdisableautofill=true