clarify behavior when both version and ensure are specified

Before this commit:

* If version is generic (absent, latest, present), the catalog
  compiles but fails to apply on host; rspec does not check this.

* If version is specified as 0.5.5 and ensure is specified,
  the version overrides ensure; rspec is undefined.

After this commit:

* If version is generic, the catalog compiles and applies;
  rspec confirms.

* Version always overrides ensure; rspec confirms.

New unit tests:

    docker
      with generic version
        version absent
          behaves like an ensure
            should contain Package[docker] with name => "lxc-docker" and ensure => "absent"
        version latest
          behaves like an ensure
            should contain Package[docker] with name => "lxc-docker" and ensure => "latest"
        version present
          behaves like an ensure
            should contain Package[docker] with name => "lxc-docker" and ensure => "present"
      version overrides ensure
        version absent and ensure absent
          behaves like an ensure
            should contain Package[docker] with name => "lxc-docker" and ensure => "absent"
        version latest and ensure absent
          behaves like an ensure
            should contain Package[docker] with name => "lxc-docker" and ensure => "latest"
        version present and ensure absent
          behaves like an ensure
            should contain Package[docker] with name => "lxc-docker" and ensure => "present"
        version 0.5.5 and ensure absent
          behaves like a custom version
            should contain Package[docker] with name => "lxc-docker-0.5.5" and ensure => "present"
        version absent and ensure latest
          behaves like an ensure
            should contain Package[docker] with name => "lxc-docker" and ensure => "absent"
        version latest and ensure latest
          behaves like an ensure
            should contain Package[docker] with name => "lxc-docker" and ensure => "latest"
        version present and ensure latest
          behaves like an ensure
            should contain Package[docker] with name => "lxc-docker" and ensure => "present"
        version 0.5.5 and ensure latest
          behaves like a custom version
            should contain Package[docker] with name => "lxc-docker-0.5.5" and ensure => "present"
        version absent and ensure present
          behaves like an ensure
            should contain Package[docker] with name => "lxc-docker" and ensure => "absent"
        version latest and ensure present
          behaves like an ensure
            should contain Package[docker] with name => "lxc-docker" and ensure => "latest"
        version present and ensure present
          behaves like an ensure
            should contain Package[docker] with name => "lxc-docker" and ensure => "present"
        version 0.5.5 and ensure present
          behaves like a custom version
            should contain Package[docker] with name => "lxc-docker-0.5.5" and ensure => "present"
use shared examples for DRYer rspec

All unit tests pass before and after this commit.