[Path Usage Unification] Plugin Installation Paths (further enhancements)

[Path Usage Unification] Plugin Installation Paths (further enhancements)

[Issue#]   LINUXWRT-422
[Problem]  A lot of paths used in wrt are constructed manually
[Cause]    N/A
[Solution] Created class PluginPath which gathers all manually constructed paths

[Verification]
    1. Build wrt-installer repo with --define "WITH_TESTS ON"
    2. Install plugins by using wrt-installer -p and
        run tests wrt-installer-tests-general --output=text
    They shoud install without problems and tests should pass.

Change-Id: I82f1f3d2e9b46b9363f93311557984da1aeaabbd