Use DevStack functions in upgrade scripts

* Source target DevStack's functions in upgrade scripts to match what the
  project config functions expect.
* Source target DevStack's lib/tls in upgrade scripts that source
  target's lib/keystone (is_ssl_enabled_service())

Change-Id: Iab78d1efb7e40a3a67408f6e9a071723a3e931ff
Mention tox as a prereq

grenade uses tox to run the tempest smoke tests, but doesn't do anything
to install it.

Change-Id: I8c23cde0f0d225384d7c40666b48107c23305bb4
Change default target from stable/icehouse to master

As of writing this commit message icehouse has yet to be released so
stable/icehouse doesn't exist.  So make the default values the same as
how we use grenade in gate.

Change-Id: I6102c67b6c0021ccd9d7ec540cc1510491f88eb2
Don't stop n-cpu if in DO_NOT_UPGRADE_SERVICES

To support the partial-ncpu grenade test, if n-cpu is in
DO_NOT_UPGRADE_SERVICES don't stop it.

Requires stable/havana devstack patch: I031707a49974df684cc473737ceb07cecc262c66

Change-Id: Ia63287ffbf36113cc99602d7bdc7ec040d252a6c
Merge "Use DevStack functions in upgrade scripts"
Merge "Don't stop n-cpu if in DO_NOT_UPGRADE_SERVICES"
Enable compute tests after upgrade in grenade

So far after upgrading openstack components there were skipped tests
marked as 'compute'. This patch enables them.

Change-Id: I86c31ce08083918fa2057d75f603aefd11c12bdb