Added '--swap' and '--ephemeral' documentation to booting from volume

Add documentation on swap volumes and ephemeral volumes to
doc/common/section_nova_boot_from_volume.xml

Closes-Bug: 1208904
Change-Id: I3eba9eeaf1876898a302ac1f7803209df145d41f
Merge "Added '--swap' and '--ephemeral' documentation to booting from volume"