smartos_imgadm : add a *basic* search option and an update function

* update_installed() (aka. imgadm update)

* avail(search=None) : outputs the last occurrence of 'string' from
'''imgadm avail'''
Merge pull request #5178 from virtua-network/imgadm_feature

smartos_imgadm : add a *basic* search option and an update function