installpkg() improvements

[phodge]

• [x] Allow yum=False to indicate that installpkg() shouldn't attempt to install the thing using that particular package manager
• [ ] Should be asking the package manager directly if a package is installed, not using haveexecutable()
• [ ] The description @property is probably broken in some scenarios
• [ ] We should try to clean up dependencies as well if we can, rather than just yum removeing the specific thing and leaving all the dependencies sitting around.