Adds spot-pricing flag for quick best effort calculation + docs

[JoshVanL]

What this PR does / why we need it:
Adds spot-pricing flag to cluster apply command. This will apply a best effort spot price to each instance pool based on an average of the last 3 days pricing + 25%.

Adds documentation for spot pricing.

fixes #451
fixes #452

Adds spot-pricing flag to cluster apply

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[JoshVanL]

/assign @mattbates
/cc @mattbates

[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	f1c7485	link	/test puppet-tarmak-acceptance-centos v1.14

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