Install python requirements directly

There are cases when an issue has been fixed upstream in a
dependency but not yet released and it is desirable to install
that version in a built image. pbr does not support such urls
in requirements files because releasing software that depends
on unreleased software is quite dangerous.

Update assemble and install-from-bindep to install -r requirements.txt
before installing the actual software so that image builds with
such requirements lines will work.

Change-Id: Ie17a66bdc4ea0609ce4e66051f659cbd829fdd5d

docker/python-builder/scripts/assemble

@@ -59,7 +59,14 @@ python -m venv /tmp/venv
 /tmp/venv/bin/pip install -U pip wheel
 
 # Install everything so that the wheel cache is populated
-# with transitive depends.
+# with transitive depends. If a requirements.txt file exists,
+# install it directly so that people can use git url syntax
+# to do things like pick up patched but unreleased versions
+# of dependencies.
+if [ -f /tmp/src/requirements.txt ] ; then
+    /tmp/venv/bin/pip install --cache-dir=/output/wheels -r /tmp/src/requirements.txt
+    cp /tmp/src/requirements.txt /output/requirements.txt
+fi
 /tmp/venv/bin/pip install --cache-dir=/output/wheels /output/wheels/*whl
 
 # Install each of the extras so that we collect all possibly

docker/python-builder/scripts/install-from-bindep

@@ -18,6 +18,13 @@ set -e
 
 apt-get update
 apt-get -y install $(cat /output/bindep/run.txt)
+# If a requirements.txt file exists,
+# install it directly so that people can use git url syntax
+# to do things like pick up patched but unreleased versions
+# of dependencies.
+if [ -f /output/requirements.txt ] ; then
+    pip install --cache-dir=/output/wheels -r /output/requirements.txt
+fi
 pip install --cache-dir=/output/wheels /output/wheels/*.whl
 
 # clean up after ourselves