'ceph -s' fails to run with "ERROR: missing keyring, cannot use cephx for authentication" even though the admin keyring exists and is readable

Issue

• When running ceph -s or any other ceph commands, the commands fail to run with the following output:

$ sudo ceph -s
2015-07-07 12:27:34.049719 7f19f1653700 -1 monclient(hunting): ERROR: missing keyring, cannot use cephx for authentication
2015-07-07 12:27:34.049723 7f19f1653700  0 librados: client.admin initialization error (2) No such file or directory

• The admin keyring exists and is readable by root:

$ ll /etc/ceph/ceph.client.admin.keyring 
-rw------- 1 root root 63 Jul  7 12:28 /etc/ceph/ceph.client.admin.keyring
$ sudo cat /etc/ceph/ceph.client.admin.keyring 
[client.admin]
    key = AQASB5NVsLbjIxAAAZ7D7SwX6MAS18G+zg2bHw==