Check if bootstrap dir is in path before removal

[beezz]

Module ddtrace.bootstrap.sitecustomize expect that the python application is started using the ddtrace-run command or that the bootstrap_dir is in sys.path see https://github.com/DataDog/dd-trace-py/blob/master/ddtrace/bootstrap/sitecustomize.py#L93

This PR prevent failed tracing initialization by checking presence of bootstrap dir in path.

How to reproduce:

minimal.py

#!/bin/env python
import ddtrace.bootstrap.sitecustomize

» python minimal.py
WARNING:ddtrace.bootstrap.sitecustomize:error configuring Datadog tracing
Traceback (most recent call last):
  File "/home/mkuffa/src/dd-trace-py/ddtrace/bootstrap/sitecustomize.py", line 93, in <module>
    path.remove(bootstrap_dir)
ValueError: list.remove(x): x not in list

While ddtrace-run minimal.py works fine.

[palazzem]

Thanks for the contribution @beezz (and welcome back as a contributor!). Let me review this PR and see if we can also test easily this behavior. Also I'll check and re-run the CI.

[palazzem]

Because we catch any kind of exception, we should provide a loaded status since we "silently pass" to avoid crashing the application.

[palazzem]

This script returns False without the given patch.

[palazzem]

Regression test added and merged. Feel free to contribute again if you have any other improvement in mind! Thank you very much @beezz!