Disable virtualbox host mounting by default

[r2d4]

Users can still set up manual mounts through vbox using the vboxmanage
command or GUI. Users can also utilized the built in minikube mount
command.

This is a bit of a hack, since the mounting is attempted in the driver, but it won't work without these extra steps in the image.

Fixes #1645

[aaron-prindle]

You had mentioned minishift supported passing flags/env vars to the driver. There seems to be one for the mount:
https://github.com/docker/machine/blob/master/drivers/virtualbox/virtualbox.go#L179-L183

[r2d4]

Looks like theres a config option for that now, I'll update this PR to just set it

[codecov-io]

Codecov Report

Merging #1646 into master will increase coverage by 0.03%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1646      +/-   ##
==========================================
+ Coverage   38.65%   38.68%   +0.03%     
==========================================
  Files          51       51              
  Lines        2667     2670       +3     
==========================================
+ Hits         1031     1033       +2     
- Misses       1455     1456       +1     
  Partials      181      181

Impacted Files	Coverage Δ
pkg/minikube/cluster/cluster.go	44.54% <100%> (+0.16%)	⬆️
cmd/minikube/cmd/start.go	16.85% <50%> (+0.37%)	⬆️

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[dlorenc]

Could we make this a flag? I think probably more users depend on this than we expect, and the mount command isn't quite ready to take over everything yet.

[r2d4]

I added a new flag which toggles the driver specific mounts for virtualbox and xhyve.