Install bundler from gems.

Previously on Debian based machines we attempted to install bundler from
apt and from gems. This worked on precise because the two package names
are different. It does not work on trusty because the package names are
the same and puppet complains that the namevar is duplicated and fails.

Fix this by removing all apt based bundler instalations. We will install
it only from gems and only on "thick" slaves. We do not need this on
devstack slaves because devstack can install it for itself if it needs
it.

Note, this changes the path to the `bundle` executable. We were using it
out of /usr/bin/bundle but a gem install will go in
/usr/local/bin/bundle on ubuntu and /usr/bin/bundle on centos. To keep
the chef jobs happy with this remove the explicit paths to bundle and
rely on $PATH to find it for us regardless of the platform.

Change-Id: Ic1f2158b923c5116f473f5d21250da69809b292d
Merge "Install bundler from gems."