Setup proxy info in the environment & no ipv6

Disable ipv6 in the image, otherwise git clone openstack commands
use ipv6 when run by jenkins (and ipv4 when run otherwise).

Also, the jobs run by jenkins need the proxy information set in
/etc/environment or injected by the jenkins build. All the other
settings used are ignored when setting up devstack-gate.
nodepool slaves are now deleted after the job
Switch to upstream jenkins slave setup

    Add python yaml package
    Add jenkins to sudoers list
    Switch to slave public key no whitespace
    Switch to upstream jenkins slave setup
Add gitpython needed by zuul-merger