Obscure the mainchainrpcpassword value in debug.log (just as with rpcpassword=****)

[opacey]

Is your feature request related to a problem? Please describe.
The elements log output (i.e. /debug.log) includes a summary of the configuration options just after it launches. These include the fields mainchainrpcpassword and mainchainrpcuser with their values shown in clear text.

Describe the solution you'd like
It would be nice to have these values obscured to prevent unintended disclosure of credentials. Elements already does this for rpcuser and rpcpassword, i.e. "rpcpassword=****", the same methodology may be appropriate.

Describe alternatives you've considered
None

Additional context
N/A