Refactor URL replacement in karaf-plugin

Refactor URL replacement in karaf-plugin

We need to be mindful of file separators when dealing with URI/URLs.
While File-based strings are using platform-dependend separator,
either '/' (Unices) or '\' (Windows et al.), file: URI scheme is
using '/' to separate path components (as per RFC8089).

Make sure we preserve the semantic context and perform proper
conversion between the two representations.

JIRA: ODLPARENT-214
Change-Id: I8d20ee1bdb4bfb3c1f675423a3849082c21fe26c
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>