Adds evacuate method to compute.api

Added decorator to check whether the vm is suitable to be evacuated.
The decorator validates that vm host is down.
The new evacuate method calls rebuild with the recreate flag and
specified target host.

This implements blueprint rebuild-for-ha

DocImpact

Change-Id: I877b29928c922fa366fb85deb85ddfc72d97daf8
Co-authored-by: Oshrit Feder <oshritf@il.ibm.com>
Merge "Adds evacuate method to compute.api"