Use on-demand yum metadata caching in cfn-init

Use on-demand yum metadata caching to avoid downloading
50MB (Fedora 19) of extra metadata that is not necessary
for correct cfn-init operation.

Reduces time to orchestration by about 23% and cpu
utilization by about 50%.

Full detailed analsys in the launchpad bug.

Change-Id: Id51d6d506d6051b5e83f550ef318f86d84f3c7a7
Closes-Bug: 1235824