Fixes [CHEF-2795] - Knife plugin loader rejects gems with 'chef' in their name

See http://tickets.opscode.com/browse/CHEF-2795

The knife/core/subcommand_loader.rb file has to avoid loading plugins from alternate versions of chef.

To do so, the method `from_old_gem?(path)` uses the following regexp:

    CHEF_FILE_IN_GEM = /chef-[\d]+\.[\d]+\.[\d]+/
    CURRENT_CHEF_GEM = /chef-#{Regexp.escape(Chef::VERSION)}/

Unfortunately, that precludes any gem containing the word 'chef' from being loaded (in particular, the cluster_chef tools.

Fix is to anchor the regexp:

    CHEF_FILE_IN_GEM = /^chef-[\d]+\.[\d]+\.[\d]+/
    CURRENT_CHEF_GEM = /^chef-#{Regexp.escape(Chef::VERSION)}/