utilize drone cli to run unit tests when installed

if drone is installed we will use drone cli to run tests rather
than building our own dockerfile etc.   If .drone.yml exists
in the user's repo then we will use that natively, if not then
we will construct one via details in the planfile.

if drone is not installed, then we'll do the testing the old
fashioned way.

Created a new local.conf variable 'SOLUM_INSTALL_DRONE' which
will install drone during stack creation if set.

Change-Id: I528929df53561e9977d1b672e4b4ed0de4a89951
Merge "utilize drone cli to run unit tests when installed"