Limit max connections in DMI stub

Limit max connections in DMI stub

DMI plugin in a real deployment could only handle a limited number
of parallel connections. From CPS-1968 of testing in a realistic
deployment, 25 max connections is chosen as a reasonable default.
This allows us to simulate rate limiting with DMI stub.

Functionality has been verified by changing the max connections and
registering 5000 CM-handles. It takes:
- 120 seconds with 8 max DMI connections
- 80 seconds with 16 max DMI connections

Issue-ID: CPS-2117
Signed-off-by: danielhanrahan <daniel.hanrahan@est.tech>
Change-Id: I6aaf43f721d6f2e6dcff45929fc9699e26980cdb