Override endpoint URL check for API version

get_volume_api_version_from_endpoint currently assumes that the
management_url will always be in the format of
http://<ip>:<port>/vX/<tenant_id>. Customers using a gateway, however,
can get a mangement_url in a different form like
https://<ip>:<port>/<resource>/vX/<tenant_id>.  This causes the
get_volume_api_version_from_endpoint to find the resource portion of the
url when looking for the Volume API version which makes the
cinder client unusable.

To resolve this I have implemented an exception handler in the case that
get_volume_api_version_from_endpoint fails.  If the exception is thrown, I
check for OS_VOLUME_API_VERSION in the environment and use the value of that
environment variable, if it is set, to determine the API version to use.

There are proposals to fix this via changes to keystone in future releases,
but this provides a way for IaaS Gateway users to proceed with Havana.

Change-Id: I8b978a6ecd87b81c6ae34938e37623ad1cc33994
Closes-Bug: 1233311