Allow for specifying nfs mount options

Fixes bug 1113042

Adds a new config option:
    nfs_mount_options=None (Default)

When not None, anything set here will be passed as -o <options> to
the mount command. The default behavior is to do exactly as we are
doing today and relying on the OS/kernel defaults.

Change-Id: I935ae6f4d96c20904b7132fe8c6dfd3cbe089e51
Flags: DocImpact
Make get_dev_name_for_instance() use stashed instance_type info

Related to blueprint no-db-compute

Change-Id: I6a8e357b69e6cfbdfa5be846b74baf6df3b8eae4
Merge "Allow for specifying nfs mount options"