Change unreasonable title
Change unreasonable title

Content of "Mistral User Guide" is all of Installation.
So I think "Mistral Installation Guide" is a better title.

Change-Id: I1b20c2f665bee21b2f35f06912beab3e7e3e93ea
Signed-off-by: pengdake <18650af38447d23679b24c3200fbf2d4cde059d4@gmail.com>

openstack/mistral,openstack/mistral