add user to pgb pool output (check_pgb_pool)

[danstoner]

Since a pgBouncer pool is tied to a user+database pair, the previous
output was less useful with large numbers of users in a single database.

Adding the user to the output makes it clear which pool is associated
with which value.

The new output looks something like:

POSTGRES_PGB_POOL_CL_WAITING OK: DB "pgbouncer" (host:dbserver) (port=5432) django_web@django=0 * django_admin@django=0 django_reporting@django=0 * websiteuser@bigdatabase=0 * user1@bigdatabase=0 *user2@bigdatabase=0 * user3@bigdatabase=0 * badly_behaved_user@bigdatabase=3 | time=0.02s time=0.02s time=0.02s time=0.02s time=0.02s time=0.02s time=0.02s