verify vCenter input fields on Settings tab

- this change allows us to validate vCenter related fields on Settings
  tab; it will not allow user to start deployment with empty values
- username field must be not empty
- password field must be not empty
- vCenter IP must be valid IPv4 address
- while here rename 'ip_regex' to 'ipv4_regex', to reflect the fact
  that this regular expression does not validate IPv6 addresses
- while here rename 'non_empty_regex' to 'non_empty_string', actually
  first name is missleading, regular expression match non empty strings,
  it does not means that RE itself is empty
- while here adjust message in wizard about "vCenter IP" field, lets
  explicitly state that we expect IPv4 address as input value

Change-Id: I60b0cbdf9a3801ba94483b1224f79786b37ce921
Closes-bug: #1314613
Merge "verify vCenter input fields on Settings tab"