Set mysql root password directly on resource

Set the mysql root password directly on the server resource, so it
does not need to be an attribute.  This keeps passwords sourced from
encrypted data bags out of node attributes. Note that as indicated
in https://github.com/opscode-cookbooks/mysql/issues/215 all recipes
will be removed from the mysql cookbook in favor of having consumers
work with the resources directly, thereby providing more flexibility
in terms of how the various configuration settings are sourced.
Including the "server" recipe is not going to be a viable option
once that has happened, so this change not only addresses the
attribute issue, but it will also make upcoming changes to the
mysql cookbook more seamless.  Note that the reference to the
default_version_for function had to be removed, because it is no
longer in the cookbook and therefore the build failed. It is no
longer necessary to set the version, as described in the README.md
file:

When [the version parameter is] omitted, it will install the
default MySQL version for the target platform.

Note that the code that sets node attributes in
spec/mysql-server_spec.rb was removed because it was interfering
with the new test and it did not seem to serve any purpose--there
were no failures after it was removed.

blueprint no-secret-attributes

Change-Id: I72be1e2131a107ddc43754bc5cf40a4ce7face95