Build retry loop for screen sessions

There is a timing window where we might lose the commands being
stuffed into screen because bash is spawning. In those cases, loop
around and try building screen sessions again.

Change-Id: I49247de06bbd59424cb10fb9a8db145907be5138
Related-Bug: #1331274
Always check keystone using http

The check to $service_port must be done using http. It's
$KEYSTONE_SERVICE_PORT that may use either the http or https protocol.

partial blueprint devstack-https
Change-Id: I8ae081e9710c54396b765210336a7de331947086
Add explicit libkrb5-dev dependency for keystone

Our Trusty test nodes do not include libkrb5-dev by default, and it is
needed to install some of the python libraries keystone uses (kerberos,
brought in by requests-kerberos).

Partial bug: #1334326

Change-Id: I1fdaa46c4590e69da82365dee578c1aa0ec95e80
Merge "Build retry loop for screen sessions"
Merge "Add explicit libkrb5-dev dependency for keystone"
Merge "Always check keystone using http"