Remove trailing slash in NODEPOOL_PUPPETLABS_MIRROR
Remove trailing slash in NODEPOOL_PUPPETLABS_MIRROR

Currently, variable NODEPOOL_PUPPETLABS_MIRROR for centos has an
extra slash character at the end. It seems this is not a problem when
using a mirror, but it fails when using puppetlabs yum repo as the
url [1] gives an 404 error, while [2] works fine.

[1] https://yum.puppetlabs.com//el/7/PC1/x86_64/repodata/repomd.xml
[1] https://yum.puppetlabs.com/el/7/PC1/x86_64/repodata/repomd.xml

Change-Id: Idf3a080322f117e62dec291f3047e9b5408813b9

openstack/puppet-openstack-integration,openstack/puppet-openstack-integration,openstack/puppet-openstack-integration