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
Merge "fix missing host when unshelving"
xenapi: post install scriptlet to rpm

A freshly installed XenServer requires some modifications in order to be
able to run OpenStack. Nova contains an rpm specification to create a
package with the dom0 plugins, however installing the rpm is not enough.
Some more actions need to be taken:
 - create /images, link to a directory within the local SR
 - setup an ssh keypair, and authenticate it with the host
This change adds a post script to the nova plugins rpm spec file, so
by installing the rpm on a XenServer, it is ready to run OpenStack, no
further manual actions needed.

Change-Id: Id57e3a3ab2782550468c6aa4c01059ceb7cc81ab