Make erlang node start up times configurable

Add two new variables which can be used to configure the number of
attempts to check erlang node availability (at start up) as well as the
interval between each check.

* edts-api-num-server-start-retries (default 20)
  The number of retries to wait for server to start before giving up

* edts-api-server-start-retry-interval (default 0.2)
  Time between each server availability check at start up

This can be useful when the ./start command takes more than 4 s (20 * 0.2s)
to complete (slow file system, ...).