Resizing a server by means of changing its flavor

Sometime we need resize the server when it is launched, such as the number of vcpu, the size of memory and the size of disk.
We can achieve this by means of changing its flavor.

Change-Id: I1ab6b61f286e951b644a2e66383ac62c6a6f887e
Implements: blueprint resize-server
Merge "Resizing a server by means of changing its flavor"