6.0 Add single path variable for each plugin

To get possibility download 3 plugin during
swarm tets we add single path variable
for each plugin:
* GLUSTER_PLUGIN_PATH
* EXAMPLE_PLUGIN_PATH
* LBAAS_PLUGIN_PATH

Change-Id: Ic06bbcef9badbe99a9b2d81627f8070d9e8964f6
Closes-Bug: #1401219