Set ruby and gem default to 1.9.1 for cookbook jobs on precise

For berkshelf 3.x and openstack cookbooks, we need precise and ruby 1.9.1.
Default for precise is ruby 1.8. Ruby 1.9 is installed on the precise
images but it's not set to the default.
Use update-alternatives to set Ruby 1.9.1 as the default.

Change-Id: I464b3e44a0ba860129f22f306c63e415fd4925ff
Closes-Bug: #1357439
Merge "Set ruby and gem default to 1.9.1 for cookbook jobs on precise"