Add stack phases to extras.d handling

Add distinct phase calls to the extras.d script handler.  This adds
hooks for these scripts to break down the installation for when order
is important.  The phases only apply to stack.sh (ie, $1 == stack) and
include 'install', 'post-config' and 'extra'.

A call to the extras scripts is also added to clean.sh.

All scripts also call the extras script with an argument of 'source'
immediately after the other lib/* files are sourced to allow the similar
setting of global and default variables.

extras.d/70-tempest.sh is present as an example of the structure
of these scripts.

Change-Id: Ic1fe522559b94d204d6c0319a2e3d23684c8d028
Merge branch 'fix-get-packages' into dt-test
Merge branch 'extras' into dt-test
Merge branch 'ldap-ad' into dt-test