Allow multiple branches to be specified by a single build parameter.

Currently specifying multiple branches as a single build parameter results in an incorrect lshistory command.

Build parameter: CLEARCASE_BRANCHES='branch1 branch2 branch3'
Plugin branches: ${CLEARCASE_BRANCHES}
Generated lshistory command:
/usr/atria/bin/cleartool lshistory -all -since 21-sep-14.18:07:53utc+0000 -fmt '\"%Nd\" \"%u\" \"%En\" \"%Vn\" \"%e\" \"%o\" \n%c\n' -branch <b>"brtype:branch1 branch2 branch3"</b> -nco vobs/myvob

With the fix, the plugin correctly generates a single lshistory command for each branch:
/usr/atria/bin/cleartool lshistory -all -since 21-sep-14.18:07:53utc+0000 -fmt '\"%Nd\" \"%u\" \"%En\" \"%Vn\" \"%e\" \"%o\" \n%c\n' -branch <b>brtype:branch1</b> -nco vobs/myvob
/usr/atria/bin/cleartool lshistory -all -since 21-sep-14.18:07:53utc+0000 -fmt '\"%Nd\" \"%u\" \"%En\" \"%Vn\" \"%e\" \"%o\" \n%c\n' -branch <b>brtype:branch2</b> -nco vobs/myvob
/usr/atria/bin/cleartool lshistory -all -since 21-sep-14.18:07:53utc+0000 -fmt '\"%Nd\" \"%u\" \"%En\" \"%Vn\" \"%e\" \"%o\" \n%c\n' -branch <b>brtype:branch3</b> -nco vobs/myvob