fixes CC-532: set zenoss as default user for zenrun; allow override with DESIRED_USER

hardcoding su - zenoss was removed per:
  https://github.com/control-center/serviced/pull/1350
zenrun now becomes zenoss user if it is run as non-zenoss

DEMO1 - show zenpack list still works:
```
# plu@plu-9: serviced -v=1 service run -i zope zenpack list
...
I1203 15:18:14.082062 02072 server.go:439] command: docker [run ... 0 su - root -c'${ZENHOME:-/opt/zenoss}/bin/zenrun zenpack.sh --service-id=bb87y4o5ur7kc3z6lmag07wr9 list']
...
ZenPacks.zenoss.ApacheMonitor (/opt/zenoss/ZenPacks/ZenPacks.zenoss.ApacheMonitor-2.1.4-py2.7.egg)
ZenPacks.zenoss.Dashboard (/opt/zenoss/ZenPacks/ZenPacks.zenoss.Dashboard-1.0.0-py2.7.egg)
ZenPacks.zenoss.DellMonitor (/opt/zenoss/ZenPacks/ZenPacks.zenoss.DellMonitor-2.2.0-py2.7.egg)
ZenPacks.zenoss.DeviceSearch (/opt/zenoss/ZenPacks/ZenPacks.zenoss.DeviceSearch-1.2.0-py2.7.egg)
ZenPacks.zenoss.DigMonitor (/opt/zenoss/ZenPacks/ZenPacks.zenoss.DigMonitor-1.1.0-py2.7.egg)
ZenPacks.zenoss.DnsMonitor (/opt/zenoss/ZenPacks/ZenPacks.zenoss.DnsMonitor-2.1.0-py2.7.egg)
ZenPacks.zenoss.FtpMonitor (/opt/zenoss/ZenPacks/ZenPacks.zenoss.FtpMonitor-1.1.0-py2.7.egg)
ZenPacks.zenoss.HPMonitor (/opt/zenoss/ZenPacks/ZenPacks.zenoss.HPMonitor-2.1.0-py2.7.egg)
ZenPacks.zenoss.HttpMonitor (/opt/zenoss/ZenPacks/ZenPacks.zenoss.HttpMonitor-2.1.0-py2.7.egg)
ZenPacks.zenoss.IRCDMonitor (/opt/zenoss/ZenPacks/ZenPacks.zenoss.IRCDMonitor-1.1.0-py2.7.egg)
ZenPacks.zenoss.JabberMonitor (/opt/zenoss/ZenPacks/ZenPacks.zenoss.JabberMonitor-1.1.0-py2.7.egg)
ZenPacks.zenoss.LDAPMonitor (/opt/zenoss/ZenPacks/ZenPacks.zenoss.LDAPMonitor-1.4.1-py2.7.egg)
ZenPacks.zenoss.LinuxMonitor (/opt/zenoss/ZenPacks/ZenPacks.zenoss.LinuxMonitor-1.3.0-py2.7.egg)
ZenPacks.zenoss.Microsoft.Windows (/opt/zenoss/ZenPacks/ZenPacks.zenoss.Microsoft.Windows-2.1.3-py2.7.egg)
ZenPacks.zenoss.MySqlMonitor (/opt/zenoss/ZenPacks/ZenPacks.zenoss.MySqlMonitor-3.0.3-py2.7.egg)
ZenPacks.zenoss.NNTPMonitor (/opt/zenoss/ZenPacks/ZenPacks.zenoss.NNTPMonitor-1.1.0-py2.7.egg)
ZenPacks.zenoss.NtpMonitor (/opt/zenoss/ZenPacks/ZenPacks.zenoss.NtpMonitor-2.2.1-py2.7.egg)
ZenPacks.zenoss.PythonCollector (/opt/zenoss/ZenPacks/ZenPacks.zenoss.PythonCollector-1.6.0-py2.7.egg)
ZenPacks.zenoss.XenMonitor (/opt/zenoss/ZenPacks/ZenPacks.zenoss.XenMonitor-1.1.0-py2.7.egg)
ZenPacks.zenoss.ZenJMX (/opt/zenoss/ZenPacks/ZenPacks.zenoss.ZenJMX-3.11.0-py2.7.egg)
ZenPacks.zenoss.ZenossVirtualHostMonitor (/opt/zenoss/ZenPacks/ZenPacks.zenoss.ZenossVirtualHostMonitor-2.4.0-py2.7.egg)
I1203 15:18:26.203247 02072 shell.go:231] Command returned non-zero exit code 1.  Container not commited.
# plu@plu-9: 
```

DEMO2 - help testid shows zenoss:
```
# plu@plu-9: serviced -v=1 service run -i zope help testid
...
I1203 15:17:46.699787 00674 server.go:439] command: docker [run ... 0 su - root -c'${ZENHOME:-/opt/zenoss}/bin/zenrun help.sh testid']
...


    This is an example template for a 'serviced run ...' command.  
    Each custom script must include a function '__DEFAULT__', which 
    defines the default action to be performed by the host when the 
    command completes and functions for each custom command that 
    requires a specific action from the host.

    Defined Commands:
        testcommit    testdiscard


uid=1337(zenoss) gid=1000(zenoss) groups=1000(zenoss)
I1203 15:17:53.352204 00674 shell.go:231] Command returned non-zero exit code 1.  Container not commited.
# plu@plu-9: 
```