Removal of _REPOSITORY global variable.

Oslo.db migration.py uses global variable to store defined
`migrate.versioning.repository.Repository` instance containing migration
repository path. At first _find_migrate_repo(repo_path) call the return value
will be stored in a global variable. So all subsequent calls
will return the existing global variable value regardless of repo_path
value passed. This design makes impossible to have multiple migration
repositories in the same project.
Detected at syncing of Keystone with Oslo.db.

Change-Id: If6187a0f7c4590e1c2cd4d998b25ee90aeac17dc
Closes-Bug: #1259229
Merge "Removal of _REPOSITORY global variable."