Use an env variable for connection string default
Use an env variable for connection string default

The user experience would be better if the connection string
can be set as an environment variable while running the trace
show command. This patch uses env[OSPROFILER_CONNECTION_STRING]
as the default connection string if set.

Change-Id: I816dceb05c4dad10897c965d7401a694d26a191a

stackforge/osprofiler,stackforge/osprofiler,openstack/osprofiler,openstack/osprofiler,openstack/osprofiler,stackforge/osprofiler