Fix: don't fallback to default stack version

[mtojek]

Bug spotted in: elastic/integrations#1877

This PR fixes bug in fallback selection of stack version. Default stack version should be selected in later processing, otherwise every ENV override var is 7.15.0-SNAPSHOT and --version is ignored.

Testing:

elastic-package stack up -v --version 7.98.0-SNAPSHOT

Error response from daemon: manifest for docker.elastic.co/beats/elastic-agent-complete:7.98.0-SNAPSHOT not found: manifest unknown: manifest unknown
Error: booting up the stack failed: running docker-compose failed: running command failed: running Docker Compose up command failed: exit status 18

[elasticmachine]

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• Start Time: 2021-10-07T12:32:32.191+0000

• Duration: 22 min 37 sec

• Commit: 228cb93

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Test	Results
Failed	0
Passed	437
Skipped	5
Total	442

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