Validate vCenter credentials on settings saving

[Igor Shishkin]

It's a good practice to validate provided data(like vCenter credentials) when user tries to save them.

Currently they're not validated and it's impossible to change them after deployment. This could cause problems and necessity to redeploy controller in case of typos or something like that.

[Dmitry Pyzhov (dpyzhov)]

Workaround should be mentioned in vCenter documentation.

[Julia Aranovich (jkirnosova)]

Just let me note that setting validation on UI can be implemented by adding a 'regex' section (http://paste.openstack.org/show/cdWhoyGC2Yu5q4fGy6Dp/) into a setting structure in the openstack.yaml configuration file. Developer should provide a source of a regular expression and an appropriate warning to display on UI.