Stop auto-detecting glance API versions

This commit switches the image api tests from auto detecting which
api versions are available to having them explicitly set in the config
file. This is to make it explicit which tests are expected to be run
instead of assuming that everything is expected to work.

Partially Implements: blueprint config-cleanup

Change-Id: Ie958a7fb03ff502c5ea1783eaae9debb442c34ea
---
 etc/tempest.conf.sample   |    9 ++++++---
 tempest/api/image/base.py |   15 ++-------------
 tempest/config.py         |   16 +++++++++++++---
 3 files changed, 21 insertions(+), 19 deletions(-)

Index: git/etc/tempest.conf.sample
===================================================================
--- git.orig/etc/tempest.conf.sample
+++ git/etc/tempest.conf.sample
@@ -587,10 +587,10 @@
 #
 
 # Is the v2 image API enabled (boolean value)
-#api_v2=true
+api_v2=true
 
 # Is the v1 image API enabled (boolean value)
-#api_v1=true
+api_v1=true
 
 
 [input-scenario]