cephadm: fix a cephadm test when docker doesn't exist

cephadm: fix a cephadm test when docker doesn't exist

Previously, when I ran the cephadm test suite locally the test case
test_mon_crush_location was failing with the error:
```
>       with pytest.raises(Exception, match='--set-crush-location
database=a'):
E       AssertionError: Regex pattern did not match.
E        Regex: '--set-crush-location database=a'
E        Input: "[Errno 2] No such file or directory: '/usr/bin/docker'"

```

At the very least this test should run on a system that doesn't
have docker installed. Although, I don't know why this test
would have passed on any system that wasn't actively running
a cephadm container...

This patch mocks out `extract_uid_gid` which was calling
out to docker even in the unit tests.

Signed-off-by: John Mulligan <jmulligan@redhat.com>