Allow to use custom mirror
Allow to use custom mirror

The current configure_mirror.sh expects no-one
want to use other than openstack.org pypi mirrors.

Having the configure_mirror.sh to use a costum,
NODEPOOL_PYPI_MIRROR instead of NODEPOOL_REGION when defined.

It allows averyone to use his local pypi mirror, with his
nodepool.

Change-Id: I6f0bd3372be09ccf423caacff83e615038c96ec5

Tesora/tesora-project-config,coolsvap/project-config,Tesora/tesora-project-config,coolsvap/project-config,anbangr/osci-project-config,open-switch/infra_project-config,openstack-infra/project-config,citrix-openstack/project-config,citrix-openstack/project-config,noorul/os-project-config,open-switch/infra_project-config,anbangr/osci-project-config,dongwenjuan/project-config,openstack-infra/project-config,noorul/os-project-config,dongwenjuan/project-config