Ability to toggle service account usage

[cjchand]

Resolves #14

Adds new value: serviceAccount.enabled.

Verified behavior is unchanged if:

1. serviceAccount.enabled = true
2. serviceAccount.enabled is unspecified or commented out

Only if serviceAccount.enabled is explicitly set to false does it remove spec.template.spec.serviceAccountName

While I manually verified across the charts locally, here are some rudimentary proof points setting:

1. Nothing - using default values
2. Setting serviceAccount.enabled to false
3. Setting Setting serviceAccount.enabled to true

 ✘ me@laptop  ~/Projects/personal/snyk-broker-helm   feature/service-account-enabled-toggle ● 
helm template snyk-broker charts/snyk-broker/. \ 
--set scmType=github.com \ 
--set brokerToken=my-broker-token \ 
--set scmToken=my-scm-token  \ 
--set brokerClientUrl=https://snyk-broker.example.com/broker | grep serviceAccountName
      serviceAccountName: snyk-broker
 me@laptop  ~/Projects/personal/snyk-broker-helm   feature/service-account-enabled-toggle ●  helm template snyk-broker charts/snyk-broker/. \ 
--set scmType=github.com \ 
--set brokerToken=my-broker-token \ 
--set scmToken=my-scm-token  \ 
--set brokerClientUrl=https://snyk-broker.example.com/broker \ 
--set serviceAccount.enabled=false | grep serviceAccountName
 ✘ me@laptop  ~/Projects/personal/snyk-broker-helm   feature/service-account-enabled-toggle ●  helm template snyk-broker charts/snyk-broker/. \ 
--set scmType=github.com \ 
--set brokerToken=my-broker-token \ 
--set scmToken=my-scm-token  \ 
--set brokerClientUrl=https://snyk-broker.example.com/broker \ 
--set serviceAccount.enabled=true \ 
--set serviceAccount.create=true | grep serviceAccountName
      serviceAccountName: snyk-broker

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