Set up virtualbox shares on start

Previously they were set up on create, but this meant that they
would not be created when boot2docker was upgraded.

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Merge branch 'set-up-shares-start' of https://github.com/bfirsh/boot2docker-cli into bfirsh-set-up-shares-start

* 'set-up-shares-start' of https://github.com/bfirsh/boot2docker-cli:
  Set up virtualbox shares on start

Also, revert the removal of the `--vbox-share` flag.
Merge branch 'bfirsh-set-up-shares-start'

* bfirsh-set-up-shares-start:
  Set up virtualbox shares on start