engine: custom properties: allow '=' in regex

In custom properties sheet when regex contains '=', we cannot parse
the string using split('=') (the parsing is invalid).
To solve that added a hard limit of 2 for the string split.

Bug-Url: https://bugzilla.redhat.com/956467
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>
Change-Id: Ic845e2ab5a0f23d48c1506f4f834a851009ea253