Fix bug introduced by #236

RM9709
RM9837

Adds support for Redis sentinels
remove test_models_sync migration test

Because these unit tests run against SQLite, it blocks
us using the more advanced features of modern, more
robust SQL data stores. Removing this test will prevent
this limitation from blocking us.
Merge pull request #241 from Cerberus98/rm9837_work

RM9837
Merge pull request #242 from TMaddox/remove_test_migrations_sync

remove test_models_sync migration test
Merge pull request #245 from asadoughi/9709_second_attempt

Fix bug introduced by #236
Ensure code will mark currently fully allocated subnets as full

    - unit test that logic is setting next_auto_assign_ip to -1 when subnet is fully allocated
    - functional tests validating the same
    - fix subnet_find_allocation_count
        - was just ordering by used ip count, instead of what is remaining.
        - example, a /31 and /32 could both have 0 ips used, but their remaining is 2 and 4, so the /31 should be first returned