utils: LocalConfig: add getProperty with an optionalSuffix String.

public String getProperty(String key, String optionalSuffix, boolean allowMissing);
Returns the value associated with key_optionalSuffix if it is defined or the one
associated with key. If both are missing and allowMissing = false throws an
IllegalStateException.

Change-Id: Ic37b6aa5ce44600fc647915b9f71632c98bbbae3
Signed-off-by: Mooli Tayer <mtayer@redhat.com>