🏃[e2e] improve and cleanup e2e clusterctl

[fabriziopandini]

What this PR does / why we need it:
This PR implements a set of small improvements and cleanup to the e2e clusterctl package:

• removes +build e2e tag (not required in the framework)
• removes inconsistency around LogPath/LogFolder variable naming (now it is used consistently LogFolder)
• refactors config.Validate into two smaller functions (fixes a linter check)
• add few utility functions to the config type

Which issue(s) this PR fixes:
xref #2753
xref #2637
xref #2636

/assign @vincepri
/assign @sedefsavas

[vincepri]

Don't we need this? @wfernandes has a different PR that adds this

[fabriziopandini]

@vincepri @wfernandes
The test framework is basically a library that CAPI is offering to external users, so IMO it should build (and run tests) like any other package

[wfernandes]

FWIW I was adding a build tag to docker/e2e/custom_management.go

[vincepri]

If we remove the build tag, make test will run also the tests in these files

[vincepri]

/approve
/milestone v0.3.4

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: fabriziopandini, vincepri

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[sedefsavas]

/lgtm