Update requirements.txt to include keystoneclient

Keystone client is required by nova client and needs to be in
requirements.txt. It is currently in test-requirements.txt but also
needs to be in install requirements.

Change Ief6cd7e752fd8c9e9157364f99e270da7faff074 introduced the
requirement for keystone client.

Change-Id: I91f58810e0511d0d4acc5bba0c0e946edf247896
Closes-Bug: 1358675