XenAPI: Fix new useage of trueorfalse

* Ensure that Xen setup scripts will continue to function when unset
  variables are used in stackrc
* Ensure that the generic functions are sourced in all places that xenrc
  (which sources stackrc) is sourced.

Change-Id: I54eba20733c2e149621b74a1387f0bef14fca12e
Begin is_service_enabled() cleanup

This converts the special cases in the is_service_enabled() function to call
individual functions declared by the projects.  This allows projects that
are not in the DevStack repo and called via the extras.d plugin to handle
an equivalent service alias.

* Ceilometer
* Cinder
* Glance
* Neutron
* Nova
* Swift

TODO: remove the tests from is_service_enabled() after a transition period

Patch Set 2: Rebased

Change-Id: Ic78be433f93a9dd5f46be548bdbd4c984e0da6e7
read_password needs to store in .localrc.auto if local.conf is used.

when running stack.sh with no passwords in local.conf
read_password() creates localrc and local.conf is ignored

Change-Id: I25ad07569d2b42b190449591d5a01ade8022392c
remove build_tempest

we haven't actually used this script in about a year, in favor of
the actual in tree lib/tempest.

Change-Id: I9d78b395846ebe833a38ba50edae226040cd7f45
Merge "read_password needs to store in .localrc.auto if local.conf is used."
Merge "XenAPI: Fix new useage of trueorfalse"
Merge "remove build_tempest"
Merge "Begin is_service_enabled() cleanup"