OpenAIAnswerGenerator not working with custom prompt_template

[spalman]

Describe the bug
class OpenAIAnswerGenerato has an error in the required params check.

haystack/haystack/nodes/answer_generator/openai.py

Line 141 in e6af353

if all(p in prompt_template.prompt_params for p in required_params):

Error message

[<ipython-input-41-fbc29f0e30ff>](https://localhost:8080/#) in <module>
      7 required_params = ["context", "query"]
      8 if all(p in lfqa_prompt.prompt_params for p in required_params):
----> 9     raise ValueError(
     10         "The OpenAIAnswerGenerator requires a PromptTemplate that has `context` and "
     11         "`query` in its `prompt_params`. Supply a different `prompt_template` or "

ValueError: The OpenAIAnswerGenerator requires a PromptTemplate that has `context` and `query` in its `prompt_params`. Supply a different `prompt_template` or use the default one.

To Reproduce

lfqa_prompt = PromptTemplate(name="lfqa",
                             prompt_text=""" 
                             Synthesize a comprehensive answer from your knowledge and the following topk most relevant paragraphs and the given question. 
                             \n===\Paragraphs: $context\n===\n$query""",
                              prompt_params=["context", "query"]) 
# Let's initiate the OpenAIAnswerGenerator 
generator = OpenAIAnswerGenerator(
    api_key='',
    model="text-curie-001",
    max_tokens=500,
    presence_penalty=0.1,
    frequency_penalty=0.1,
    top_k=1,
    temperature=0.3,
    prompt_template=lfqa_prompt
)```


**System:**
 - Haystack version (commit or version number): 1.13.2

[FateDawnLeon]

Seems like this is a bug, where on the 8th line should be if not all(p in lfqa_prompt.prompt_params for p in required_params):

[sjrl]

Thanks for catching this. Opened a PR with the fix.