Give -p option to install.sh if we are bootstrapping a pre-release version

One of the main problems around [CHEF-4911](https://tickets.opscode.com/browse/CHEF-4911) is best described by @sean-horn:

We always see this around upgrade time with prerelease/nightly versions of chef-client floating around:  The scenario:  Person installs a prerelease version of chef-client on their workstation. Person tries to bootstrap a new node from that same workstation. Bootstrapping blows up with a 404 when retrieving the chef-client package, because omnitruck only returns prerelease versions in the metadata when the &prerelease=true option is given on the omnitruck metadata URL. The knife chef-full bootstrap template only looks at &prerelease=false&nightlies=false packages.

I believe we can workaround this by passing `-p` to `install.sh` if the bootstrap version is a pre-release version. But it's totally possible that I'm missing something.

/cc: @opscode/client-eng 

PS: I'm not suggesting to close CHEF-4911 after this fix.