Making is_ubuntu func available to fetch_devstacks

When the fetch_devstacks function runs as part of grenade.sh
it tries to install devstack tools. As part of the devstack tools
installation, python3 gets installed through a function called
install_python3 which uses a function called is_ubuntu to
determine if the system is ubuntu. However this is_ubuntu
function is not available at that moment, so the script fails.

This commit sources functions-common from the devstack directory
which includes is_ubuntu so the grenade.sh script does not break.

Change-Id: I92b9f731e99df0840be296bfa8ce6a70e02a1f91
Closes-bug: 1659081

grenaderc

@@ -8,6 +8,11 @@ if [ -f $RC_DIR/localrc ]; then
     source $RC_DIR/localrc
 fi
 
+# When Python 3 is supported by an application, adding the specific
+# version of Python 3 to this variable will install the app using that
+# version of the interpreter instead of 2.7.
+export PYTHON3_VERSION=${PYTHON3_VERSION:-3.5}
+
 # Base GIT Repo URL
 # Another option is http://review.openstack.org/p
 # Another option is https://github.com

inc/bootstrap

@@ -272,6 +272,7 @@ function fetch_devstacks {
 
     # This depends on REQUIREMENTS_DIR being set in grenaderc, which
     # it needs to be to have gotten this far.
+    source $TARGET_DEVSTACK_DIR/functions-common
     source $TARGET_DEVSTACK_DIR/inc/python
     install_devstack_tools