[Path usage Unification] Plugin Installation part unification

[Path usage Unification] Plugin Installation part unification

[Issue#] LINUXWRT-395
[Problem] A lot of paths used in wrt are constructed manually
[Cause] N/A
[Solution] Used module DPL::Utils::Path
[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: I6e03f4732a6bb10df78dfb0bc451f5a3da627ac8