AWS secret engine - fix generate_credentials for STS endpoint

[briantist]

Fixes #930
Closes #932

The generate_credentials endpoint was not properly passing parameters when calling the sts endpoint, so all parameters were being ignored.

The method was also using the PUT verb, even though the docs specify POST. According to the Vault documentation this was ok because for the time being, the Vault API considered PUT and POST to be equivalent, however I've updated the method to use POST anyway.

Changes to generate_credentials:

• Use _adapter.post() instead of _adapter.put()
• Remove name from the parameters passed to the endpoint
• Send the parameters to the endpoint properly (with the json parameter)
• Add the role_session_name parameter to the method
• Update documentation for the method

The addition of role_session_name means we'll release this in the next minor version after it gets merged.

[sunchill06]

LGTM. Thanks for taking care of this.

[codecov]

Codecov Report

Merging #934 (d70938d) into develop (026d6b5) will increase coverage by 0.21%.
The diff coverage is 50.00%.

@@             Coverage Diff             @@
##           develop     #934      +/-   ##
===========================================
+ Coverage    81.34%   81.55%   +0.21%     
===========================================
  Files           65       65              
  Lines         2969     3003      +34     
===========================================
+ Hits          2415     2449      +34     
  Misses         554      554              

Impacted Files	Coverage Δ
hvac/api/secrets_engines/aws.py	96.15% <50.00%> (ø)
hvac/api/auth_methods/mfa.py	100.00% <0.00%> (ø)
hvac/api/secrets_engines/kv.py	100.00% <0.00%> (ø)
hvac/api/secrets_engines/identity.py	94.00% <0.00%> (+0.02%)	⬆️
hvac/utils.py	77.27% <0.00%> (+0.20%)	⬆️
hvac/adapters.py	86.73% <0.00%> (+0.27%)	⬆️
hvac/api/vault_api_category.py	95.12% <0.00%> (+0.67%)	⬆️
hvac/v1/__init__.py	85.71% <0.00%> (+1.89%)	⬆️

[briantist]

thanks for reviewing @dhuckins !