Add base tests for plugins

Add test for lbaas plugin:
For each plugin test variable PLUGIN_PATH should be exported

I.* deploy master node
  * upload plugin on it using path expoerted in PLUGIN_PATH var
  * install plugin
  * enable it
  * deploy cluster with  neutron
  * check that lbaas agent is running
  * create pool/vip
  * assert that vip in Active state
  * run OSTF
By default neutron vlan is used, if we need gre here
variable NEUTRON_SEGMENT_TYPE should be exported with gre value

II. * Deploy neutron with lbaas
    * Reset cluster and add node
    * redeploy and check lbaas

Add template for plugin_example
* deploy neutron simple with example plugin
* deploy ha nova with example plugin
* deploy ha neutron with example plugin
  add nodes, re-deploy cluster
  verify that plugin works on added nodes

III. glusterfs plugin
Deploy simple
  * Deploy simple anv with plugin
  * verify plugin health

Deploy HA:
 * Deploy ha env with plugin
 * Verify health
 * add 2 nodes
 * Re-deploy env
 * verify health
By default nova net is used, to get neutron
Variable NEUTRON_ENABLE. In this case neutron vlan wiil be deployed.
Also here we should export GLUSTER_CLUSTER_ENDPOINT in
format like ip:/volume1/subvolume

Change-Id: Ib19864d777f0439cd2ba172a8a9bd14abe09a201