Reorder ref count tests to ensure correct checks on count of volume users

[govint]

Had made these changes in PR #811, will submit via this PR instead as its not related to adding the photon driver.

Plus, the ref count test code is using plugin logs to figure out test result which doesn't work as expected always. These fixes ensure that the test result is deterministic.

Changes are:

1. Got errors when creating the test volume that volume size is not specified - which happens as a result of a failing check in the auth code. Passing a size of 1GB for the test volume used.

2. Moved the test for user count on the test volume (via check_files) to after the check on the users of the test volume. The test for user count itself runs a countainer to check the number of users of the ref count test volume and ends up increasing the volume user count. And the subsequent check (on user count) failed on occasion. Hence the reorder.

Test:

1. Ran the ref count test manually and it works ok.

sudo ./build/docker-volume-vsphere.test
Running tests on  unix:///var/run/docker.sock (may take a while)...
Running parallel tests on unix:///var/run/docker.sock and unix:///var/run/docker.sock (may take a while)...
PASS

[govint]

Fixes #790

[govint]

Those issues I've addressed in this change that actually repro'ed during testing. Pls. review and let me know if its good to merge.

[pdhamdhere]

LGTM