radosgw-admin: add --uid check in bucket list command

[vumrao]

radosgw-admin: add --uid check in bucket list command
Fixes: https://tracker.ceph.com/issues/41589

[vumrao]

• The vstart cluster results:

[root@3cfea3e7c7e2 build]# bin/radosgw-admin user list
[
    "56789abcdef0123456789abcdef0123456789abcdef0123456789abcdef01234",
    "testx$9876543210abcdef0123456789abcdef0123456789abcdef0123456789abcdef",
    "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef",
    "test",
    "testid"
]

• Not providing value to the argument --uid

[root@3cfea3e7c7e2 build]# bin/radosgw-admin bucket list --uid=
bad uid

• Providing a user which does not exist

[root@3cfea3e7c7e2 build]# bin/radosgw-admin bucket list --uid=xyz
ERROR: could not find user: xyz

• User with buckets

[root@3cfea3e7c7e2 build]# bin/radosgw-admin bucket list --uid=testid
[
    "my-new-bucket1",
    "my-new-bucket2",
    "my-new-bucket3",
    "my-new-bucket4",
    "my-new-bucket5"
]

• User without buckets

[root@3cfea3e7c7e2 build]# bin/radosgw-admin bucket list --uid=test
[]

[mattbenjamin]

lgtm

[vumrao]

• After Matt suggestion test result from vstart cluster:

# bin/radosgw-admin bucket list --uid=
no value for uid

[cbodley]

passed qa in http://pulpito.ceph.com/cbodley-2019-09-18_14:51:41-rgw-wip-cbodley-testing-distro-basic-smithi/

(slow ops failures tracked in https://tracker.ceph.com/issues/41834)

[vumrao]

Thank you @cbodley.