Hide "Download EC2 Credentials" if EC2 is missing

If EC2 is not a registered service, don't show the "Download EC2 Credentials"
button in the "API Access" tab.

Fixes bug 1331153

Change-Id: I4e600b49573d2d3d96e93c984346d23f02e45e28
Merge "Hide "Download EC2 Credentials" if EC2 is missing"