refactor(settings): make HathorSettings always return the same instance

[jansegre]

This wasn't exactly the case before this commit. On one side when using the deprecated module settings the same module would always be loaded, and the same SETTINGS property would be accessed, but reloading the module (or at least leaving that up to importlib) unnecessary. On the new YAML system the file is reloaded everytime, this is also unnecessary.

This commit will make it the last HathorSettings instantiated will be returned, as long as the source is the same, otherwise an exception is raised.

There is a little small caveat that is fixed in this commit is that in the YAML file could in theory be altered between different calls to HathorSettings, which would lead to different content being loaded silently. What happens now is that the first time it is loaded is what is used for every call, any change in the file won't have any effect.

Acceptance Criteria

• always return the same hathor.conf.HathorSettings instance when calling hathor.conf.get_settings.HathorSettings
• replace get_settings_module with get_settings_module_path that returns the loaded path instead of the loaded module

[codecov]

Codecov Report

Merging #626 (94e99cc) into master (7f35116) will increase coverage by 0.02%.
The diff coverage is 65.71%.

@@            Coverage Diff             @@
##           master     #626      +/-   ##
==========================================
+ Coverage   83.91%   83.94%   +0.02%     
==========================================
  Files         240      240              
  Lines       20127    20126       -1     
  Branches     2744     2743       -1     
==========================================
+ Hits        16890    16895       +5     
+ Misses       2640     2635       -5     
+ Partials      597      596       -1     

Impacted Files	Coverage Δ
hathor/conf/get_settings.py	70.00% <63.63%> (+15.00%)	⬆️
hathor/builder/cli_builder.py	70.74% <100.00%> (-0.16%)	⬇️

... and 4 files with indirect coverage changes