Avoid use of JOpt's #defaultsTo method

Providing an explicit default using the #defaultsTo method ends up
short-circuiting the Spring Environment's hierarchical property resolution
process. for example, if --port.useManualPortForwarding has a default
value of `false`, then the command line property source always returns a
value when its #getProperty method is invoked by the Environment. This
means that a lower-precedence property source never has the opportunity
to return its value. For example, if port.useManualPortForwarding had
been set to true in the filesystem property source
(at ${user.data.dir}/bitsquare.properties), this property value would
never be resolved because the default command line property source always
overrides it (thus the notion of "short circuiting" above).

This change eliminates the use of JOpt's #defaultsTo method in favor of
a simple approach to advertising default values (if any) in the option's
description string. The result is --help output that reads exactly the
same as it did before, but no actual default value is set at the command
line property source level.

Note that the default property source is still created, and default
values are still assigned in BitsquareEnvironment#defaultPropertySource.
This property source has the lowest precedence, and this means that any
and all other property sources have the opportunity to provide a value
and override the default.