A change further up in the stack revealed that "ERROR" wasn't
accepted as a log mask string at the engine level.

Signed-off-by: Kris Jacque <kris.jacque@intel.com>

Allow daos_agent to optionally handle unresolvable client
uids via custom mapping. In deployments where the agent
may not have access to the same user namespace as client
applications (e.g. in containerized deployments), the
client_user_map can provide a fallback mechanism for
resolving the client uids to known usernames for the
purpose of applying ACL permissions tests.

Example agent config:

credential_config:
  client_user_map:
    default:
      user: nobody
      group: nobody
    1000:
      user: joe
      group: blow

Features: control
Required-githooks: true
Change-Id: I72905ccc5ddee27fc2101aa4358a14e352c86253
Signed-off-by: Michael MacDonald <mjmac@google.com>

On heavily-loaded client nodes where many processes are being
launched by the same user or users, the admin may optionally
enable the credential cache in the agent in order to lower
agent overhead caused by generating identical credentials
for each process owned by a user. The agent-generated
credential is presented by the client process during pool/container
connection and is used to evaluate ACL permissions for
that connection.

Example config:
credential_config:
  cache_lifetime: 1m

Features: control
Required-githooks: true
Change-Id: I6ae2a8be1dd97ef14e0ccef0283d65bc1fabc4ed
Signed-off-by: Michael MacDonald <mjmac@google.com>