Add required permission for the vCenter service account

The VMware compute driver needs 'Sessions/Validate session' permission
in order to check if a given session is active. This patch adds this to
the permissions table.

Change-Id: Ice8d2b4a02a6c92253332f39d412119e6be7dde9
Closes-Bug: #1339440
Merge "Add required permission for the vCenter service account"