Cleaning up the entry point into nightly builds. With this change, a single spec...

Cleaning up the entry point into nightly builds. With this change, a single spec can be used to specify multiple variations of a build type, e.g.:

marcs_trunk_build = {
	'tags':'planetlab-tags.mk',
	'distro':['centos5','f8'],
	'personality':['linux32','linux64'],
	'test': 0,
	'release':['k22','k27']
}

Specifies 8 builds.