Don't enable services implicitly #106

Describe the bug
A clear and concise description of what the bug is.
When you don't enable a service, it still may be loaded.
To Reproduce
Steps to reproduce the behavior:
Use the next config:

pyms:
  services:
    metrics: true
  config:
    debug: true
    app_name: alertmanager-actions
    testing: false

When receiving a request, it will return the next error:

Tracer error: 'NoneType' object has no attribute 'tracer'

Expected behavior
It should not return that error.

Additional context
I'm using pyms 2.4.1 version.

When disabling explicitly the service, it does not return the error:

pyms:
  services:
    metrics: true
    tracer:
      enabled: false
  config:
    debug: true
    app_name: alertmanager-actions
    testing: false