feat(api/deploy): Default to redeploy if to_revision is not set

[kaustavha]

Allow automatic deploys without specifying to revision by making it default to current revision of that stack thats been deployed.

I noticed that in staging there's a specific revision that's been deployed over and over again, this should make that process easier.

Note: This PR does not make dreadnot auto-deploy to the latest git revision, only the git revision that's already been deployed and who's sha1 hash can be retrieved by running stack.getSummary.

[mburns]

Needs to be rebased/linted, but then I'm :+1:

[mburns]

fixed. :+1:

[jguice]

/opt/dn-bundle-4245263c548c2c7e052bce48ec63bced1db11011/lib/web/handlers/api.js:123
    getWarning: getWarning,
                ^
ReferenceError: getWarning is not defined
    at Object.exports.getAPIHandlers (/opt/dn-bundle-4245263c548c2c7e052bce48ec63bced1db11011/lib/web/handlers/api.js:123:17)
    at Object.exports.registerAPI (/opt/dn-bundle-4245263c548c2c7e052bce48ec63bced1db11011/lib/web/app.js:109:34)
    at Object.exports.run (/opt/dn-bundle-4245263c548c2c7e052bce48ec63bced1db11011/lib/web/app.js:167:27)
    at /opt/dn-bundle-4245263c548c2c7e052bce48ec63bced1db11011/lib/entry.js:57:12
    at /opt/dn-bundle-4245263c548c2c7e052bce48ec63bced1db11011/node_modules/async/lib/async.js:721:13
    at /opt/dn-bundle-4245263c548c2c7e052bce48ec63bced1db11011/node_modules/async/lib/async.js:52:16
    at /opt/dn-bundle-4245263c548c2c7e052bce48ec63bced1db11011/node_modules/async/lib/async.js:269:32
    at /opt/dn-bundle-4245263c548c2c7e052bce48ec63bced1db11011/node_modules/async/lib/async.js:44:16
    at /opt/dn-bundle-4245263c548c2c7e052bce48ec63bced1db11011/node_modules/async/lib/async.js:718:17
    at /opt/dn-bundle-4245263c548c2c7e052bce48ec63bced1db11011/node_modules/async/lib/async.js:167:37

Might have to revert this unless someone wants to pop in with a fix pretty quick ;)