Clear out the $DIR_EXECUTABLE variable so we catch the situation when we
lose the directory with binaries after first sapinstance_init
invokation. The second sapinstance_init invocation will not detect it as
it will have already preset the $DIR_EXECUTABLE from previous run.
This may allow us running actions after second sapinstance_init
invocation that would be not run if we knew that we miss the needed
binaries.

This affect the following functions/operations:

• sapinstance_monitor_clone
• sapinstance_promote_clone
• sapinstance_demote_clone

This is limited to only Master/Slave resource and only when the
sapinstance_init is invoked multiple times during one operation.