Fixes percona::server recipe on rhel.

- Unsets pid file for rhel so uses default. RPM packages seem to start
  the service automatically, and can't switch location afterward unless
  we modify mysql::server directly to shut down service before writing
  my.cnf template.
- Moves password setting from suite to platform for tests.
- Uses chef-rewind on rhel to set '--disablerepo' flag on yum_package
  resources for native CentOS repos.  This acts as a decent substitute
  for apt_preference pinning, which doesn't have an analog.
Clarifies travis.yml file.