build-appliance-image: include proxy on pip3 installation

Otherwise, we get timeouts, leading to errors:

    pip._vendor.requests.exceptions.ConnectTimeout: HTTPSConnectionPool(host='pypi.python.org', port=443): Max retries exceeded with url: /pypi/pip/json (Caused by ConnectTimeoutError(<pip._vendor.requests.packages.urllib3.connection.VerifiedHTTPSConnection object at 0x7ff741bb3ef0>, 'Connection to pypi.python.org timed out. (connect timeout=5)'))
    WARNING: TOPDIR/tmp/work/qemux86-poky-linux/build-appliance-image/15.0.0-r0/temp/run.do_populate_poky_src.185123:1 exit 1 from 'pip3 install --user -I -U -v -r TOPDIR/tmp/work/qemux86-poky-linux/build-appliance-image/15.0.0-r0/rootfs/home/builder/poky/bitbake/toaster-requirements.txt'
    DEBUG: Python function do_image finished

Complete log at [1]

[1] http://errors.yoctoproject.org/Errors/Build/43393/

(From OE-Core rev: 8291156c30e54a05ad455dda2b8540a570bb0f4a)

Signed-off-by: Leonardo Sandoval <leonardo.sandoval.gonzalez@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-core/images/build-appliance-image_15.0.0.bb

@@ -100,7 +100,11 @@ fakeroot do_populate_poky_src () {
 	export STAGING_INCDIR=${STAGING_INCDIR_NATIVE}
 	export HOME=${IMAGE_ROOTFS}/home/builder
 	mkdir -p ${IMAGE_ROOTFS}/home/builder/.cache/pip
-	pip3 install --user -I -U -v -r ${IMAGE_ROOTFS}/home/builder/poky/bitbake/toaster-requirements.txt
+	pip3_install_params="--user -I -U -v -r ${IMAGE_ROOTFS}/home/builder/poky/bitbake/toaster-requirements.txt"
+	if [ -n "${http_proxy}" ]; then
+	   pip3_install_params="${pip3_install_params} --proxy ${http_proxy}"
+	fi
+	pip3 install ${pip3_install_params}
 	chown -R builder.builder ${IMAGE_ROOTFS}/home/builder/.local
 	chown -R builder.builder ${IMAGE_ROOTFS}/home/builder/.cache
 }