Replace `--omnibus-version` with `--bootstrap-version`

Compatible with `knife bootstrap`, `knife ec2 server create`, etc.
The version specification is still only used with Omnibus bootstrap
method.
Always install 'chef' gem

When using gem installation method, we want to install the 'chef' gem.
Still allow different distros to choose other gems to be installed.

This is also preparation for specifying the wanted Chef version.
Use `--bootstrap-version` also for gem install method
Move Omnibus options construction to KnifeSolo::Bootstraps

Consistent with the gem installation method.
Merge pull request #185 from tmatilai/bootstrap-version

Replace `--omnibus-version` with `--bootstrap-version` and use it also with gem installers