Spice proxy config setting to be read from the spice group in nova.conf

All spice configuration should be read from the spice group
in the nova config. As it's currently being done in the nova.spice
module.
Renaming spicehtml5proxy_{host, port} parameters to
html5proxy_{host, port}

DocImpact
Change-Id: I36c014c8b8a0796cda494f9450a1fc30104504a1
Closes-Bug: #1308418
Merge "Spice proxy config setting to be read from the spice group in nova.conf"