Thanks dhan. Looks like your solution worked. I was able to deploy the Cloudera Manager (haven't tried using it yet though) - just that the install succeeded.
I have a few notes for the other community folks -
1. Instead of the image URL dhan provided, I used the following URL -
This seems to be the latest RHEL 6 image.
Checking the My Console > Compute Engine > Images > <open the image link> > Equivalent Rest link gives you the image path.
2. I was monitoring the /var/log/cloudera-director-server/application.log during this install process.
When saving the instance template with the above mentioned image URL, I noticed this log entry which says -
[2016-02-17 19:18:20] INFO [qtp239217021-45] - c.c.d.g.c.GoogleComputeInstanceTemplateConfigurationValidator: path parameter: Invalid path 'google.compute.imageAliases.https://www.googleapis.com/compute/v1/projects/rhel-cloud/global/images
/rhel-6-v20160211': Token not allowed in path expression: ':' (you can double-quote this token if you really want it here)
However, if you actually try to use "" around the image URL, some sort of validation fails and the UI does not allow you to save the instance template.
Ignoring the log (i.e. using the full URL without the quotes) however seemed to be innocuous (for now).