Fix base currency option handling

[cbeams]

This change fixes #2048 by removing the assignment of a default value
for the baseCurrencyNetwork option at the level of the command line
option parser. The assignment of this default was an oversight in #1961
(specifically commit 83e1dd3) that did not account for the fact that
users can change the baseCurrencyNetwork value via the Settings screen
in the application. When users change the setting in the application, the new
value is persisted to /bisq.properties, which is handled at
runtime as a PropertySource with lower precedence than the command line
property source, which means that the changed value is never picked up
because the higher-precedence command line PropertySource always has a
default value.

This fix is surgical in that it addresses only this specific option. A
subsequent change should address the more general issue that setting
defaults in the command line option parser always precludes the
possibility of overriding them in bisq.properties. Basically, we should
revert to the previous strategy of reporting what the default value will
be in the help text without actually assigning a default value in the
option parser using the defaultsTo method.

[ManfredKarrer]

utACK

[ManfredKarrer]

Thanks @cbeams for fixing it so fast! @freimair I merge it already so that users who run from source can this fixed. But please have a look if we need to change that for the other options as well. At least with the path for log files there was an issue as well (which is fixed by not using getProperty).