CLOUDSTACK-7153: more robust box cleanup

Having experimented with many edge cases of running multiple build.sh
commands in parallel / against busy virtualbox setups, the only really
reliable way to produce consistent images is to not do these commands
in parallel and to not do them while the machine is doing many other
things.

If virtualbox or the machine that hosts it is very busy, and/or it has
a lot of disks it knows/knew about, and/or its tuesday, behavior may
be a bit different.

Realizing this reality, this commit adds some scripts that try really
hard to set virtualbox back to known/healthy state before building.
CLOUDSTACK-7153: more reliable support for vagrant box export

Veewee supports exporting vagrant boxes out of virtualbox, out of the box.
However, it assumes that it can export a disk if the shutdown of the vm that
is using that disk has succeeded. This assumption is not strictly always true
(see previous commit). So, we replicate the bit of logic in veewee for making
vagrant boxes.

This has the added side benefit of creating an .ovf export only once, rather
than once for vmware and then again for vagrant.
CLOUDSTACK-7153: try to fix jenkins problems with last 2 commits

* bundle install needs to run before running the vbox cleaning scripts,
  so move prepare step before clean step
* feature branches have / in their name which is a bad character to
  put into filenames
CLOUDSTACK-7153: remove erroneous backticks

The backticks in the Vagrantfile template were getting evaluated by bash.
This caused some harmless but confusing error messages to appear on running
the build. Easy fix is to remove them.
Merge branch 'feature/systemvm-refactor' into sbp-develop