Prompt user to add Kerberos keytab entries as needed in NFS form.

adds a new API call to generate the required kerberos SPN entries for making NFS + kerberos work correctly.

A rough idea of how this would work is prompt user to add kerberos SPN entry for NFS after NFS form update in following conditions:

1. v4_krb changes from False to True
2. call to "kerberos.keytab.has_nfs_principal" returns False
3. AD state state is "HEALTHY". (you can call "directoryservices.get_state" and parse output)

After form update, if user accepts prompt, present with minimal form like the "AD leave" one. New API call is "nfs.add_principal",

@accepts(
Dict(
'add_nfs_principal_creds',
Str('username', required=True),
Str('password', required=True, private=True)
)
)
async def add_principal(self, data):

When NFSv4 is enabled we should also maybe have a button appear at the bottom of the screen like the "LEAVE" button in the AD form, that brings up the same prompt.

The button can be greyed out / disabled if "kerberos.keytab.has_nfs_principal" returns True.