Don't directly pip install tempest pip-requires

This code:

    # Tempest doesn't satisfy its dependencies on its own, so
    # install them here instead.
    pip_install -r $TEMPEST_DIR/tools/pip-requires

causes us to do:

    Found existing installation: oslo.config 1.1.0
     Uninstalling oslo.config:

so rather than 1.1.0 installed, we end up with 1.1.0b1 and everything
goes kabloomy.

It looks like this comment came from a time (commit 9a352daf) before
tempest included proper install_requires (tempest commit 5abb253c3).

It should be safe to delete this and allow setup_develop() do all the
work just like the other projects.

Change-Id: I869f727900bb08c72e5e8f4e84858a9c77049715

lib/tempest

@@ -266,10 +266,6 @@ function configure_tempest() {
 # install_tempest() - Collect source and prepare
 function install_tempest() {
     git_clone $TEMPEST_REPO $TEMPEST_DIR $TEMPEST_BRANCH
-
-    # Tempest doesn't satisfy its dependencies on its own, so
-    # install them here instead.
-    pip_install -r $TEMPEST_DIR/tools/pip-requires
 }
 
 # init_tempest() - Initialize ec2 images