Allow processors in root PG, fix json literal block and controller service regex

[ghost]

Trying to import this template results in a number of errors related to:

• processors in the root process group. The import of templates with processors in the root process group fails because such a process group is internally know as 'NiFi Flow' but cannot be reached by that name through the API. This change fixes this issue by setting, depending on the pgName, the correct path in _changeProcessGroupState().
• literal json blocks are not correctly accepted with toPrettyString(). I replaced those calls with toString(). Look for the '|-' string in sample_template.yml for examples of where this error occurs.
• the controller service name regex lookup fails when a function name is used in the name. For instance a name like ${event_type:toLower():equals('ad')} cannot be looked up and will result in an error. I changed the regex to accept any name, except ones that hold :, ( or ) characters.

This is my first attempt at using Groovy, and my first ever pull request, so please let me know if there's anything missing/wrong :-).

[aperepel]

Hey @jvanbemmelen thanks for this PR! I will take a look at these in the next few days. Your groovy-fu is not bad at all, keep them coming :)