fix missing host when unshelving

An instance has the hostname of the host on which it is running.
The hostname is deleted when shelving. However, the hostname is not
recreated when unshelving.

This patch sets the hostname when unshelving.

Change-Id: I2e431718198321c46b9335e6fb7ab7be3943fab6
Closes-Bug: #1237868
(cherry picked from commit 9532e4ed969e47826801b14aff40dd9e356f2677)
Merge "fix missing host when unshelving" into stable/havana