Adds flag to use own AWS key pair

[JoshVanL]

What this PR does / why we need it:
Adds a flag to use an existing key pair in AWS.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #312

Release note:

NONE

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[JoshVanL]

/cc @dippynark

[JoshVanL]

/assign @dippynark

[JoshVanL]

/assign @simonswine

[simonswine]

I think this should be a configuration option in the tarmak.yaml for the aws provider (and should have the option to be overriden in AWS specific sections of environment,clusters or instancePools)

/unassign
/assign @JoshVanL

[jetstack-bot]

@JoshVanL: PR needs rebase.

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[jetstack-bot]

@JoshVanL: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
tarmak-puppet-module-tarmak-acceptance-1-14-centos	41657c3	link	/test puppet-tarmak-acceptance-centos v1.14

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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