Allow setting LOG_FILE_PATH when starting Stellar-Core in captive mode

[bartekn]

What problem does your feature solve?

Some users would like to specify where Stellar-Core stores it's log file files because a default /tmp location is not always right.

What would you like to see?

A new config flag that is forwarded to Captive Stellar-Core config (both online and offline modes).

What alternatives are there?

Ideally, Stellar-Core team implements stellar/stellar-core#2943 so there is no need to store log file anywhere (we embed it in Horizon log anyway).

[bartekn]

OK, there was a misunderstanding on my side. We still need a flag so nothing changes in this issue but if the flag is not set we should explicitly set LOG_FILE_PATH="" so the log file is not written anywhere.

[Shaptic]

After discussion with ops, here's the agreed-upon strategy for this moving forward:

• We'll expose an optional parameter, e.g. --captive-core-log-path
• If this is set, we'll stop logging subservice=core in Horizon and pass the path along to LOG_FILE_PATH
• If not, we'll continue with the current behavior (log in Horizon and set param to "")