Issue 49618 - Set nsslapd-cachememsize to custom value
    
    Description:
    Added function for getting available memory.
    Added test case to set nsslapd-cachememsize to custom value above 3805132804 bytes. Test is skipped if available memory is lower than we want to set.
    
    https://pagure.io/389-ds-base/issue/49618
    
    Reviewed by: vashirov, spichugi (Thanks!)