Remove redundant word from OS::Nova::Server

All parameters not marked as "Required" are optional. The documentation
generated from the schemas will already note this, so having the word in
the description is redundant.

Change-Id: I1df10c598969e97d5c1c8ecd1c2896a59ae1d2d1
Closes-Bug: #1225845
Merge "Remove redundant word from OS::Nova::Server"