Remove juju.Conn and consolidate some testing code

Part of the journey to cleaning up state and environ dependencies.

juju.Conn is no longer used. juju/conn.go and juju/conn_test.go are deleted. Some remaining functionality from conn is moved to juju/testing/conn.go, and also apiconn_test.go. Of the code moved to testing/conn.go, there is still potentially unneeded conn functionality - this will be removed next.

Test suites which used to have a State and an Environ and a Conn now only have a State and an Environ.
conn_test.go included tests for DeployService, so these tests were moved to deploy_test.go where they belong. 

Tested live on ec2 - deployed a unit, ensured ha.
Looks like this was intended to be `defer apiConn.Close()` which we probably do want?