vars: Add site to list of extra cluster properties (fate#316118)

Also clean up and clarify use of "advanced" parameters -- that is,
parameters that should not be included in tab completion list.
Merge pull request #17 from krig/fate316118

vars: Add site to list of extra cluster properties (fate#316118)
clidisplay: Remove singleton for CliDisplay
term: Turn TerminalController into a module
cache: Turn WCache singleton into module
idmgmt: Turn IdMgmt singleton into module
schema: Convert schema singleton to module
msg: ErrorBuffer doesn't need to be a singleton
report: Report doesn't need to be a singleton
cibconfig: CibFactory doesn't need to be a singleton
cibstatus: Remove usage of Singleton
singletonmixin: Remove the singleton mixin as it's no longer used
dev: Remove reference to singletonmixin
cibconfig: Eliminate external use of is_cib_sane()