Fix: use two Docker images of elastic-agent

[mtojek]

Issue: #510

This PR modifies logic responsible for selecting Elastic-Agent images:

For stack_version < 7.15.0 - elastic-agent
For stack_version >= 7.15.0 - elastic-agent-complete.

[elasticmachine]

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• Start Time: 2021-09-22T13:47:09.703+0000

• Duration: 30 min 51 sec

• Commit: 59c539e

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Test	Results
Failed	0
Passed	433
Skipped	5
Total	438

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[mtojek]

self-approved, emergency fix