webadmin: Improve wording of the memory optimization options

Our memory optimization radios in Cluster / Optimization
referenced memory page sharing. Since KSM is not the only
technology we use to achieve this it was confusing our users.

The text is changed to refer to physical memory overcommit.

Change-Id: If1242b0b2314ff615a02baa98ecd58421c500b42
Bug-Url: https://bugzilla.redhat.com/1090576
Signed-off-by: Martin Sivak <msivak@redhat.com>