This commit adds support to install the Ironic Prometheus Exporter,
it's necessary to set `enable_prometheus_exporter=True` or pass
`--enable-prometheus-exporter` when using the `bifrost-cli`
Also the ipe is now enabled in the
bifrost-integration-tinyipa-keystone-ubuntu-bionic/centos8
Change-Id: I3972e88162fba31ad6e262f47b67a37cd37a2076