Fix CDN and Fastpurge client handling

[rbikar]

Previously CDN or Fastpurge client were instatiated even if there was not enough configuration provided for them.

Also the edgerc conf. file was required everytime even if we didn't want to use cache purging functionality.

Let's now fix that so we get clients only if at least some minimal configuration is provided.

[rbikar]

run tests

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (c0a4cc6) 96.86% compared to head (0671ace) 96.87%.

:exclamation: Current head 0671ace differs from pull request most recent head 8c79207. Consider uploading reports for the commit 8c79207 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #210 +/- ## ======================================= Coverage 96.86% 96.87% ======================================= Files 8 8 Lines 990 991 +1 ======================================= + Hits 959 960 +1 Misses 31 31 ``` | [Impacted Files](https://app.codecov.io/gh/release-engineering/ubi-population-tool/pull/210?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=release-engineering) | Coverage Δ | | |---|---|---| | [ubipop/\_cdn.py](https://app.codecov.io/gh/release-engineering/ubi-population-tool/pull/210?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=release-engineering#diff-dWJpcG9wL19jZG4ucHk=) | `98.74% <100.00%> (+<0.01%)` | :arrow_up: |

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[rbikar]

Jenkins CI passed as well, not sure why it's not reported back to GH now.