VMM: VirtualMailManager.__pwhash() added support for 'doveadm pw'.
dovecotpw was replaced by `doveadm pw` in Dovecot v2.0. To use doveadm
instead of dovecotpw use the following settings in vmm.cfg:
[bin]
dovecotpw = /path/to/doveadm
[misc]
dovecotvers = 20