Retry topo read with timeout to avoid test flakiness.

The test_no_mysql_healthcheck was sometimes failing to find the mysql
port in the topo entry. There's no guarantee that the topo has been
updated as soon as the tablet reports being in SERVING state, so we
may need to wait and re-read the topo entry.
Merge pull request #98 from youtube/tabletmanager-test

Retry topo read with timeout to avoid test flakiness.