Added functionality for blue-green releases on deploy

[tirimia]

At work, we found ourselves implementing blue-green releases in every pipeline, and I thought I could just contribute it to levant so everyone can do it in a more convenient way, and give something back to a great FOSS project.

[jrasell]

@Atnar thanks for this and apologies for the delay in looking. There is currently a merge conflict within command/deploy.go; would you be able to take a look at this?

[tirimia]

Thanks for looking at it! Should be fine now ;)

[hashicorp-cla]

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1 out of 2 committers have signed the CLA.

• [x] tirimia
• [ ] Theodor-Alexandru Irimia

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[tirimia]

@jrasell , can you please help me fix this whole CLA thing ? I signed the agreement but it seemingly picked up my account twice.