Update virtualenv, pip and git existent check in cli-ref tool
Update virtualenv, pip and git existent check in cli-ref tool

In the cli auto generation tool default,
it uses /usr/bin only for virtualenv, pip and git
But ubuntu server installs these two commands to
/usr/local/bin.

This patch searches these commands by the which command.
If it cannot find these commands in the search path,
it reports error.

Closes-Bug: #1515878
Change-Id: I61ed42d27724a5abb72748beb58c1c012c52a3f6

openstack/openstack-doc-tools,openstack/openstack-doc-tools