Allow overriding solum endpoint defined in the service catalog.

When authenticating against keystone, the service catalog
returned may or may not contain an endpoint for solum url.
It should be possible for users to override the solum endpoint
url when an environment variable is set (SOLUM_URL).

Change-Id: I16f6fb7d7a79fc82a469b4d71d1aa4b9b2ca5bc8