Build and test managed plugin support in CI

[msterin]

Fixes #886
Depends on #1019

Per offline disussion, we plan to do the following in the CI:

• after vDVS is build, we deploy the VIB, then select a (ubuntu) VM
• copy ./plugin folder and ./build/docker-volume-vsphere file there
• run 'cd ./plugin; make info clean plugin' there. This will build the plugin locally
• to sanity test on the same VM:
• Enable plugin
• create a vsphere volume
• run busybox and write something to this volume
• run busybox and check the written stuff
• check the volume indeed belongs to the plugin (Not local)
• rm the volume
• disable the plugin and remove it (or rm -rf or just make clean in ./plugin)