Improve user home detection

[jordeu]

Description

On some environments the user home it is not correctly discovered and it returns a ? instead of the real path. We can use the same approach already implemented to detect the user, first check if HOME environment variable exists, otherwise fallback to System.getProperty("user.home").

Example

$ ./tw-agent 8e2e618d-2bc2-4d82-8e20-407ce0db6c08
11:16:46.738 INFO - Established active environments: [cli]
11:16:46.753 ERROR- The work directory '?/work' do not exists. Create it or provide a different one using '--work-dir'.

NOTE: A non-static linked version of tw-agent do not have this problem.

[jordeu]

Related issue #45