Fixes lp1634770: openstack bootstrap panics with no version specified.

[anastasiamac]

We panic on bootstrap, when openstack identity URL has no version.
For example, given cloud definition (from the bug):

nuc03-openstack:
type: openstack
auth-types: [userpass]
regions:
RegionOne:
endpoint: http://192.168.20.218:5000/

QA:

1. Setup openstack cloud definition without version in identity URL
2. juju bootstrap nuc03-openstack
3. [no panic]

As a drive-by, we also check if version specified was well-formed.
Any of the variations of these following forms of identity URL should produce user-friendly message helpful to correct endpoint URL:

• "https://keystone.internal/a"
• "https://keystone.internal/v"
• "https://keystone.internal/V
• "https://keystone.internal/V/"
• "https://keystone.internal/100"
• "abc123"
• "https://keystone.internal/vot"

[anastasiamac]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/9546

[anastasiamac]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/9548

[anastasiamac]

!!build!!

[anastasiamac]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju