config: allow independent rpccookie config

The bitcoind .cookie contains an autogenerated user (__cookie__) and
password (random string), which can be used instead of the rpc user name
and password. This commit allows for running against bitcoind without
having to access bitcoin.conf like in the case for pure
user/password/zmq configuration.

docs/release-notes/release-notes-0.15.1.md

@@ -115,6 +115,9 @@
 * [Re-initialise registered middleware index lookup map after removal of a 
   registered middleware](https://github.com/lightningnetwork/lnd/pull/6739)
 
+* [Bitcoind cookie file path can be specified with zmq
+  options](https://github.com/lightningnetwork/lnd/pull/6736)
+
 ## Code Health
 
 ### Code cleanup, refactor, typo fixes
@@ -135,6 +138,7 @@
 
 # Contributors (Alphabetical Order)
 
+* bitromortac
 * Carsten Otto
 * Elle Mouton
 * ErikEk