Allow password-protected CAs to call revoke command

[isemaya-square]

This addresses #110

We test this in workflow_test.go, which is an integration test. We check to make sure that calling revoke does not result in any errors.

I also added password as a flag to the revoke command (mainly for integration testing purposes - this is a common pattern in workflow_test.go).

Manual Testing: I ran go build for certstrap so that I can run certstrap with the updates in this PR - the successful case is with the Alice cert, and the failure case (i.e. we fail to supply the passphrase when revoking) with Bob):

~/Development/certstrap isemaya/certstrap-fix-revoke-key-for-passphrase-protected-ca ./certstrap init --common-name "CertAuth"
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Created out/CertAuth.key (encrypted by passphrase)
Created out/CertAuth.crt
Created out/CertAuth.crl
~/Development/certstrap isemaya/certstrap-fix-revoke-key-for-passphrase-protected-ca ls
CODEOWNERS      Dockerfile      NOTICE          certstrap       cmd             go.mod          out             tests
CONTRIBUTING.md LICENSE         README.md       certstrap.go    depot           go.sum          pkix
~/Development/certstrap isemaya/certstrap-fix-revoke-key-for-passphrase-protected-ca ./certstrap request-cert --common-name Alice
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Created out/Alice.key
Created out/Alice.csr
~/Development/certstrap isemaya/certstrap-fix-revoke-key-for-passphrase-protected-ca ./certstrap sign Alice --CA CertAuth
Enter passphrase for CA key (empty for no passphrase): 
Created out/Alice.crt from out/Alice.csr signed by out/CertAuth.key
~/Development/certstrap isemaya/certstrap-fix-revoke-key-for-passphrase-protected-ca ./certstrap revoke --CN Alice --CA CertAuth
Enter passphrase for CA key (empty for no passphrase): 
~/Development/certstrap isemaya/certstrap-fix-revoke-key-for-passphrase-protected-ca ./certstrap sign Alice --CA CertAuth
Certificate "Alice" already exists!
~/Development/certstrap isemaya/certstrap-fix-revoke-key-for-passphrase-protected-ca ./certstrap sign Robert --CA CertAuth
Get certificate request error: stat /Users/isemaya/Development/certstrap/out/Robert.csr: no such file or directory
~/Development/certstrap isemaya/certstrap-fix-revoke-key-for-passphrase-protected-ca ./certstrap request-cert --common-name Bob
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Created out/Bob.key
Created out/Bob.csr
~/Development/certstrap isemaya/certstrap-fix-revoke-key-for-passphrase-protected-ca ./certstrap sign Bob --CA CertAuth
Enter passphrase for CA key (empty for no passphrase): 
Created out/Bob.crt from out/Bob.csr signed by out/CertAuth.key
~/Development/certstrap isemaya/certstrap-fix-revoke-key-for-passphrase-protected-ca ./certstrap revoke --CN Bob --CA CertAuth
Enter passphrase for CA key (empty for no passphrase): 
get CA key error when saving revoked certificates: x509: decryption password incorrect