Fixes README.md on APACHE_ENABLED_SERVICES=keystone

The documentation stated that, to enable Keystone running on Apache,
one should set:
APACHE_ENABLED_SERVICES+=keystone,swift

However, it actually works with:
APACHE_ENABLED_SERVICES+=key,swift

Change-Id: I8cc92e9426d7fc3117b30df01babdb50576328af
Closes-bug: #1311461
Merge "Fixes README.md on APACHE_ENABLED_SERVICES=keystone"