HostKSM: Detect missing pgrep and log an error

If pgrep is missing on the system the HostKSM Collector will silently
fail and MOM will exit.  This is just a CollectionError so catch the
condition and raise the proper exception.

Change-Id: Iae678edaac5560d0066ebde1a0da6f610e48fb02
Signed-off-by: Adam Litke <alitke@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22087
Reviewed-by: Piotr Kliczewski <piotr.kliczewski@gmail.com>
Reviewed-by: Martin Sivák <msivak@redhat.com>