ncm-accounts: Validate shell against /etc/shells

[Gargravarr2112]

It is possible to specify an invalid path to a shell for a user on a configured machine, either with a typo or a shell that isn't installed. Linux includes /etc/shells for a list of valid shells on the local system, which local account management will check against. It may be useful to do the same in NCM.

• The version of the component: ncm-accounts 19.12.0-1
• What OS version: Scientific Linux 7.9
• How to reproduce: Include accounts configuration and create a new account with an invalid path to a shell
• Actual result: Invalid shell is accepted and inserted into /etc/passwd on the local system
• Expected result: Error is reported that the shell does not exist on the local system