Expose the ttl and install_cache properties on CachingObtainer.

They're "public" anyway, in the sense that they're passed into
__init__(). But this allows the PantsObtainer subclass of CachingObtainer
 to access them in a natural way and use them to initialize the temporary
CachingObtainers it delegates to.

Reviewed at https://rbcommons.com/s/twitter/r/267/

Signed-off-by: John Sirois <jsirois@twitter.com>