Check host exists before evacuate

Before an evacuate is attempted, there is no checks in
place to see if the host exists. This
results in rpc call being made to non existent host.

This patch adds a simple check to both v2 and v3 extensions
to ensure that the compute service exists before we attempt
to evacuate.

Fixes bug 1208369

Change-Id: I07103ecb8310d8a891ee5fb21a1f8aa7427e0d78
Merge "Check host exists before evacuate"