Increase glance_num_retries for Glance and Cinder

Sometimes but very often, nova-compute fails to start a VM when using a
large RAW image from Glance (RBD driver).

After looking in the code, it looks like we could increase the number of
retries when downloading an image from Glance.

Closes-bug #505
Merge pull request #506 from enovance/glance-timeout

Increase glance_num_retries for Glance and Cinder