This fetches the astara administrative API endpoint via keystone
instead of creating one from a pre-configured networking prefix.
Partial-bug: #1516787
Partially-implements: blueprint astara-horizon-mitaka
Depends-on: I2b96137c05b832a68ad01a11ec0cfb2371111c3c
Change-Id: I5879a2fb9e93c981196f55d1f298c137f315590d