Removed current working directory logic retrieving test file resources.

Removed current working directory logic retrieving test file resources.

Some of the SNAPS tests leverage file resources contained within the
project. These resources were being accessed via relative paths which
required the test clients to ensure that the CWD was the <repo>/snaps
directory. Replaced that logic to leverage the import pkg_resources
making the tests much more flexible and robust.

JIRA: SNAPS-89

Change-Id: Ic9c429ee53e4dd785641e11e1ed4de5aeeab54d1
Signed-off-by: spisarski <s.pisarski@cablelabs.com>