auto-disable Nexus anonymous mode only if not explicitly enabled

The postStart.sh currently sets "enabled": false if no ENABLE_ANONYMOUS_ACCESS environment variable is set, regardless its value.
Additionally ENABLE_ANONYMOUS_ACCESS seems not used anywhere.
All this makes no sense. Moreover "setting ENABLE_ANONYMOUS_ACCESS (to any value) disables the anonymous access" ! it does the contrary of its meaning.

Fix this to something more consistent.
Expected behavior:

• if the JSON contains a value for "enabled", then keep it unchanged
• if the JSON contains no value for "enabled", then a value must be set
• the default value is false
• determine what to do with ENABLE_ANONYMOUS_ACCESS: is it worth to keep this variable, then it could behave as "if set, then overwrites the JSON with its value (false OR true)".

Related changes:
> e06d2b8 2019-11-27 Alexis Timic NXBT-2896: fix jq
> 09a6041 2019-11-21 Julien Carsique SUPINT-1574 add jq

• 09a6041: jq was simply not installed missing
• e06d2b8: the fix keeps the JSON valid and it uses a boolean value instead of string. This repairs the JSON parsing

https://github.com/nuxeo/docker-nexus3/blob/master/postStart.sh#L67-L71