Correct the Inconsistencies in Tacker CLI help texts.

Some of the tacker CLI command help texts are inconsistent, some help messages start with capitals and some with small alphabets, This bug should aim to correct all the help texts of CLI Commands and make them uniform with other openstack projects.

Example:

 tacker vnfd-create [-h] [-f {html,json,shell,table,value,yaml}]
                          [-c COLUMN] [--max-width <integer>] [--noindent]
                          [--prefix PREFIX] [--request-format {json,xml}]
                          [--tenant-id TENANT_ID]
                          (--vnfd-file VNFD_FILE | --vnfd VNFD) [--name NAME]
                          [--description DESCRIPTION]

Create a VNFD.

optional arguments:
  -h, --help show this help message and exit
  --request-format {json,xml}
                        The xml or json request format
  --tenant-id TENANT_ID
                        The owner tenant ID
  --vnfd-file VNFD_FILE
                        specify vnfd file
  --vnfd VNFD specify vnfd
  --name NAME Set a name for the vnfd
  --description DESCRIPTION
                        Set a description for the vnfd

output formatters:
  output formatter options

  -f {html,json,shell,table,value,yaml}, --format {html,json,shell,table,value,yaml}
                        the output format, defaults to table
  -c COLUMN, --column COLUMN
                        specify the column(s) to include, can be repeated

table formatter:
  --max-width <integer>
                        Maximum display width, 0 to disable

json formatter:
  --noindent whether to disable indenting the JSON

shell formatter:
  a format a UNIX shell can parse (variable="value")

  --prefix PREFIX add a prefix to all variable names