vpn.data, bond.options, and user.data encode their values directly as
keys in keyfile. However, keys for GKeyFile may not contain characters
like '='.

We need to escape such special characters, otherwise an assertion
is hit on the server:

  $ nmcli connection modify "$VPN_NAME" +vpn.data 'aa[=value'

Another example of encountering the assertion is when setting user-data key
with an invalid character "my.this=key=is=causes=a=crash".